3.0 - First pass at making Email\Email use Multipart classes.

There is still work to be done around a more generic multipart builder, but I think that might require a separate Mime building sub-package.  I'm not sure how far down that road we might want to go.  If people think its worthwhile, or would lead to cleaner internals I can see how hard it would be to build a simple multipart message generator. Parsing messages is more difficult and something I don't think we really need in CakePHP.