added support for PGP/MIME encrypted emails.

I've added basic support for PGP-MIME based on [RFC 3156](http://tools.ietf.org/html/rfc3156). I have not been able to get the K-9 application to build with either IntelliJ or Android Studio, so I have been unable to test this. However, it appears to be correct based on my limited understanding of the code base.

The lack of support for PGP-MIME is very frustrating as it causes the workflow for a PGP-MIME email to be: select attachment -> view attachment -> select all text -> switch to APG app -> paste text -> type passphrase... etc. This should enable the viewing of PGP-MIME encrypted emails inline, just as if they were a normal PGP encrypted email.

Thanks.