Functionality to enable k-9 to decrypt and display RFC 2440 and RFC 3156

Functionality to enable k-9 to decrypt and display RFC 2440 and RFC 3156 conforming messages (MIME-type: application/pgp-encrypted) in k-9 with the help of Android Privacy Guard (APG). The main difference is that the RFC-compliant emails use attachments to store the pgp encrypted text in either us-ascii or base64. The RFC aproach is for example used in Apple Mail.

This is a first approach, which is working for normal and a RFC-compliant emails. I am still working on reading through the RFCs which means there may be some corner cases left unimplemented. 
In addition, the decrypted text is currently just put back into k-9 mail without respecting the encodings such as iso-8859-1 or utf-8 which leads to incorrect display of special characters, e.g. German umlauts.