Add clone support

This pull request adds clone support to ruby_bugzilla using the xmlrpc interface directly to the bugzilla server because the python-bugzilla interface is unable to access/set the clone related fields.

I had investigated allowing additional fields to be set when cloning, for example setting flags, however flags can not be set using the xmlrpc interface when "creating" the new clone. Flags must be set with a modify operation.  I decided to replicate the clone functionality provided by the Bugzilla Web interface which does not copy flags from the original bug ticket and expects the user to set the flags in a separate "modify" step.

@fryguy please review