The new --old-version option allows one to point JSDuck at doc exports
for different versions.  JSDuck will then import the exported data and
generate @since tags based on that.

For start only classes get @since tags.

The Class#get_members now allows three values for the 'static'
parameter: true, false, nil.  False can now be used to grab only
instance members, while nil will return both instance & static.

When expanding widget.* enums, set enum properties as private
when the class defining the alias is private.

It was at the wrong level - foo.* enums were reported
as empty.

So one can declare:

    @enum [xtypes=widget.*]

and it will be automatically populated with all available widget types.

Auto detected properties default to being private normally, but for
enums, all possible values should be public.

    /** @enum {String} */
    Foo = {foo: 1, bar: 2, baz: 3};

It acts just like class with special enum=true flag.

The values of an enum are just class properties.

This will automatically choose an available Markdown implementation.

Additionally added missing require statement to doc_formatter_spec.

A regression. This worked in JSDuck 3.x but the behavior was lost
during parser updates.  Now additionally covered with unit tests.

This premature optimization was causing a strange test failure
regarding MetaTagRegistry.  I did some speed measurements and
there seemed to be no performance benefit of having this class
being singleton.

Add esprima.js also to include files.

When using NodeJS as the JS engine, the serialization of RegExp object
results in an empty Object/Hash not in nil as in therubyracer gem.

ExecJS will select the available JavaScript runtime.

- Move JS/CSS parsing error handling up to the level where we read the
  file in, so when IO error occours we'll also catch that.

- Handle guide reading/formatting errors.

- Handle errors in formatting class docs (wasn't able to test it, but
  a user reported a stack trace with encoding error happening inside
  there - maybe a difference when running in Windows).

Previously when default value was 'false' it got ignored completely
as a result of just checking the truthiness of default value.  Now
explicitly checking that the value is not nil.

When parent class has config "foo" then the auto-detected property
"foo" in child class should also be a config.

Incorporated this logic in InheritDoc class.
Added separate test file.

It seems that VBulletin membergroupids have changed.  ID 7 no more
means a moderator - instead all moderators have 2 and dev team
has additional ID of 19.

The mapping of membergroupids to moderator status is now done just
once at login time.  Previously the group number was checked whenever
one wanted to determine moderator status, although a boolean moderator
field had been set at login.

Additionally fixed emailHash saving in ForumUser#clientUser - previously
the result of calculation was just thrown away.

Solved conflicts with @throws implementation and addition to CSS parser.