Move CFPropertyList to a more idomatic layout

Currently if a user were to want to use CFPropertyList without installing it
via rubygems, the most common installation would have been into site_ruby. This
would result in a large number of files in site_ruby for a single project,
which is unusual. Ruby projects generally have a single ruby file in
site_ruby/vendor_ruby and a directory in site_ruby/vendor_ruby with the
remainder of the classes and modules. This commit updates CFPropertyList to
follow that structure, with cfpropertylist.rb under lib and the remainder of
the classes and modules under lib/cfpropertylist. The entry point to the
library remains as easy as `require 'cfpropertylist'`.
Update tests for new lib structure

The previous commit updated the lib structure to move most of the library under
lib/cfpropertylist. This means that the requires in the tests need to be
updated to find the files correctly. The updated requires presume that the lib
directory of cfpropertylist is in the load path of the ruby running the test
suite.
Update files listing for rdoc and gem creation

In a previous commit, the lib structure of the project was updated to move most
of the library to lib/cfpropertylist. This commit updates the gem file list and
the rdoc file list to add the files under lib/cfpropertylist.