Split Overrides and Ext Overrides tests.

These are two different things.
Fix auto-inheriting of overrides.

Overrides shouldn't be inherited, but the code in InheritMembers
happened to inherit them along with all the other things, causing
auto-detected members to share the same overrides array, resulting
in bogus lists of overridden members.

This fix should take care of bug #465.
Include Ruby version into output of --version.

Helpful for debugging purposes.
Up version to 5.3.3.
Fix +/- default of warning types in --help=warnings.

A regression introduced by the previous enhancement of Basic warning
class.
Eliminate the need to list version in gemspec.

Extract it from the run of jsduck --version.
Move JSDuck version number to separate file.

So it's easy to find and update in there.
Create Rake bump task to update version number.

rake bump

    Bumps the minor version number by one.

rake bump["6.0.0"]

    Bumps to the specified version.

The version.rb file gets updated and committed.  The commit gets tagged
with the same version number.
Fix Changelog link in README.
Refresh the list of contributors in README.

List all the contributors + those who have posted 5 or more issues.
Up version to 5.3.4.
Fix crash when parentless sub-cfg in class doc-comment.

Nothing really good to do in this situation, but at least avoid
crashing.

Refs #475
Typo in warning message.