Overhaul database schema and abstraction layer

by cketti for K-9 Mail

The goal of the project is to be able to store the structure of arbitrary complex MIME messages in the database for easy access and to be able to reconstruct a byte-identical copy of the original message. This is needed for common email encryption systems like PGP/MIME and S/MIME; but also for seemingly simple tasks like export of a single message or viewing the message source. Additionally the database abstraction layer will be changed to better utilize the help Android provides with it's API.