QPID-4567: Moved the gem packaging to using a gemspec file.

The gemspec already existed, but now the Rakefile has been removed.
Cleaned up what is included in the gemfile: removed the spec tests,
cucumber tests and examples from the gem, added the ChangeLog file.
QPID-4568: Fixed up the README.rdoc file

Removed extraneous pieces of information, such as the copyright notice
and author name. Fixed the instructions for how to generate the Swig
wrapper code and build the gem, how to install it and where to find more
information.