Lots of changes

This is a series of commits that provide several new features.

* Add the ability to check version numbers in all the features (load_class, is_class_loaded, etc), like Class::MOP does - load_class( 'Foo', { -version => 42 } )
* Small code cleanups, including switching tests to use done_testing
* Try to load Class::Load::XS, a new distro I created which takes the XS bits out of Class::MOP and packages them separately - see http://git.urth.org/?p=Class-Load-XS.git;a=summary
* Added a load_first_exting_class sub, taken more or less from Class::Load
* Copied a few tests from the CMOP test suite that tested weird corner cases for this functionality

This is all in service of having Class::MOP (and Moose) use Class::Load rather than provide its own version of these features.