The use of #skip_white must have been a mistake.

Instead call it at the top level method, ensuring it will be
called always after parsing any tag.

Just using the add_tag method like previously, but the actual
parsing of the @tag is performed in parse_at_tag method before
control flow reaches the method for parsing the concrete tag.

The code should be now both cleaner and faster (we're no more calling
look(/regex/) repeatedly to decide which at_tag method to call).

Remove bunch of duplication by parsing similar tags with a more
generic functions.

Also avoid repeating regexes by not testing for the exact tagname
any more after the first initial check.  The new helper method
parse_tag_as() replaces in lots of places two lines of code with
just one.

Suppose we're using JSDuck to generate guides from the following directory
structure:

somedir/
  guides.json
  guides/
    getting_started/
      README.md
      some-image.png
    nested/
      first_guide/
        README.md
        one.png
      second_guide/
        README.md
        one.png

In JSDuck's generated HTML, image tags in the Getting Started guide will point
to the correct path relative to the page (e.g.
'guides/guide_getting_started/some-image.png'). However, the nested guides will
specify the wrong paths to images: JSDuck will write
'guides/first_guide/one.png' instead of the correct
'guides/guide_nested_first_guide/one.png'.

This patch fixes the issue by generating the image's path based on the full
name of the guide, instead of using 'File.basename' to incorrectly crop the
path.

The strings can now contain the ] or } char without causing the
parsing to end at that point.  For example:

    @cfg {String} [foo="]"]

Also stars for classes with new members.

External classes can be now specified as patterns. For example
'Foo.*' to ignore all classes in Foo namespace.

It's now ignored together with all other possible tags within code
example blocks.

In Ruby 1.9 string[x] returns a string but in Ruby 1.8 it returns
a char code.  Fixed by using sting[x,y] which always returns a string.

One can now write doc-comment for event before the this.fireEvent()
and the event name will be picked up automatically, without any need
to add @event tag.

When encountering an unsupported @tag, JSDuck now prints a warning.

To achieve this, I've enhanced the doc-comment parser to only accept
@tags when they have a space in front of them.  Previously this would
have been detected as @method tag:

    /** foo@method.com */

Now it's treated as plain text as it should be. Only when separated
with spaces, the @method tag gets detected:

    /** foo @method .com */

Previously --ext-namespaces only applied to detecting Ext.define.

For backwards compatibility with old ExtJS sources that contain
@override to denote a method is overriding another one in parent.

One can now just add an empty doc-comment before Ext.override and
all the overrides will get detected:

    /** */
    Ext.override(Some.Class, {
        foo: 1,
	doSomething: function(){}
    });

That is, allow to use the @override tag without accompaning @class
tag to give a name for the override.

An off-by-one bug which prevented one from placing a comment right
before some other code construct without leaving a space in between.