Fixes Issue 1224

Implement DEFLATE compression for IMAP communication, enabled by
default.  User can disable compression for Wi-Fi, Mobile, or Other
networks, if it causes problems or if uncompressed communication is
faster, which is possible on Wi-Fi and wired networks, especially.

"Other" is to allow for the Android platform to introduce new
networking types without having to immediately change K-9 Mail.
However, as those arise, new network types should be added as explicit
types in K-9 Mail.