added some testcases for MimeMessage parsing + some simple refactorings ...

I would also like to have PGP/MIME support in K9, therefore I had a look a this branch. As far as I understand changing the internal structure of the Body and Part hierarchies so that the original content is still available is a prerequisite for PGP/MIME. I also think that these classes should be simplified (or at least better documented) before changing them. Therefore I started writing some unit tests and small refactorings.

Before doing some bigger refactorings I would like to assure that there are no other major pending changes around (there are quite some people who said they would work on this issue and forked the repo, but I couldn't see a lot of activity in the main line). If there are other people active, we should try to integrate often.