Fixes #460

A simple check that running jsduck --version results in 0 exit code.

Fix typo in README.md

Placed it in the same group with @deprecated and @removed, so one
can also add a version number (since when the feature was added),
and a comment:

    @experimental 3.2 To try out new Google API.

Tweaked the styles of @deprecated and @removed too, so they all look
a bit better.  To allow for different message text in @experimental,
had to introduce @msg and @since instance variables to the DeprecatedTag
base class.

Fixes #239

Grouping the --processes, --cache and --cache dir inside it.

Refs #446

Marshal dump format can change between Ruby versions and although
the Marshal has its own version numbers, I get an encoding error
when reading the Ruby 1.8 Marshal dump with Ruby 1.9.

The parsed source code data structure can also change between
versions of JSDuck.

To eliminate these problems, a .cache/manifest.txt file is created
with the contents like:

    Ruby: 1.9.3, JSDuck: 5.2.0

The file is then checked to match the current version of Ruby and
JSDuck.  If the versions don't match, the whole cache is invalidated
(all files removed from it).

Refs #446

Take the file name also into account when calculating the MD5 hash
of a cache entry.  Otherwise files that are renamed would also be
read from cache, resulting in the old filename being referenced in
the docs and also in the warning messages.

Refs #446

We remember each cache entry that was read or written during the
current jsduck run.  All others get deleted.  This ensures the .cache
directory doesn't grow over time.

Refs #446

By default the caching is disabled.  When turned on with --cache,
the caching dir will be <output-dir>/.cache/ unless overridden
with --cache-dir option.

Previously the whole output dir got cleaned up, but now it can
contain a .cache dir, which we'll want to preserve.  So I created
OutputDir class to take care of output directory cleanup in a way
that preserves the .cache/ directory inside it.

Refs #446

Refs #450

At the moment the cache is just saved to .cache/ in current dir.

The parsed data is serialized into binary format using Marshal,
which correctly preserves all the data types and is additionally
really fast.

Refs #446

Refs #444

Only data properties can have the writable flag, accessor properties
can't.

Fixes #444

Force RKellyAdapter spec source to ASCII encoding.

But only when documented with a doc-comment.  Completely ignore
getters/setters when auto-detecting class members.

Fixes #441

Make the RKellyAdapter correctly adapt the getters/setters into
properties of "get" and "set" kind.

Fixes #436

This way when an actual error happens, we don't catch it with the
generic 'rescue Exception => e' block.

Create four warning classes to handle a particular set of warnings:

- Warning::Basic -- the usual warnings :tag, :link, :global, ...
- Warning::Nodoc -- the :nodoc warning type together with its
  parameters.
- Warnings::Deprecated -- the old :no_doc* warnings which delegate to
  the new :nodoc warning.
- Warning::All -- composite warning that allows enabling/disabling all
  warning types together.

Then inside Warning::Registry delegate #set and #enabled? calls to these
warnings.

The new nodoc takes parameters, but at the moment the parsing of
more than one parameter doesn't work yet.

Perform missing docs reporting in separate class.

Test the warnings system through the Warnings class instead.
Switch Logger class back to the old system of including Util::Singleton.

Make the Logger testable by defining a LoggerCls and instantiating
it as global Logger - allowing us to instantiate the LoggerCls
separately in tests.

Test the enabling/disabling of warnings.

To avoid the no_doc_param warning.

Fixes #435

Fixes #429

Something goes wrong when we execute this BOM-stripping code in
Ruby 1.8 which has encodings support.  Didn't care to digg into this,
and so I'm just limiting this fix to Ruby 1.8 where it's actually
needed.

An improvement over the previous fix by Brian Moeskau.
Saving on the horizontal space by removing the left margings
altogether.

Cosmetic margin fix for categories page