Initial GCM registration
Initial client support for GCM message send/receive
Major storage layer refactoring to set the stage for clean GCM.

1) We now try to hand out cursors at a minimum.  There has always been
   a fairly clean insertion layer that handles encrypting message bodies,
   but the process of decrypting message bodies has always been less than
   ideal.  Here we introduce a "Reader" interface that will decrypt message
   bodies when appropriate and return objects that encapsulate record state.

   No more MessageDisplayHelper.  The MmsSmsDatabase interface is also more
   sane.

2) We finally rid ourselves of the technical debt associated with TextSecure's
   initial usage of the default SMS DB.  In that world, we weren't able to use
   anything other than the default "Inbox, Outbox, Sent" types to describe a
   message, and had to overload the message content itself with a set of
   local "prefixes" to describe what it was (encrypted, asymetric encrypted,
   remote encrypted, a key exchange, procssed key exchange), and so on.

   This includes a major schema update that transforms the "type" field into
   a bitmask that describes everything that used to be encoded in a prefix,
   and prefixes have been completely eliminated from the system.

   No more Prefix.java

3) Refactoring of the MultipartMessageHandler code.  It's less of a mess, and
   hopefully more clear as to what's going on.

The next step is to remove what we can from SmsTransportDetails and genericize
that interface for a GCM equivalent.
First cut at group messaging refactor.
Make MMS more asynchronous and consistent with new SMS types.

1) We now delay MMS notifications until a payload is received,
   or there's an error downloading the payload.  This makes
   group messages more consistent.

2) All "text" parts of an MMS are combined into a second text
   record, which is stored in the MMS row directly rather than
   as a distinct part.  This allows for immediate text loading,
   which means there's no chance a ConversationItem will resize.

   To do this, we need to include MMS in the big DB migration
   that's already staged for this application update.  It's also
   an "application-level" migration, because we need the MasterSecret
   to do it.

3) On conversation display, all image-based parts now have their
   thumbnails loaded asynchronously.  This allows for smooth-scrolling.
   The thumbnails are also scaled more accurately.
Locally encrypted messages no longer show as ciphertext notifications.
Minor refactoring
MMS and Group Messaging Improvements

1) Display the individual sender name in a group conversation.

2) Add an "address" column to MmsDatabase and keep FROM there.

3) Remove all blocking operations from MmsDatabase.Reader path.

4) Strip SMIL and other undisplayable parts from part count.

5) Fix places where messages weren't being correctly decrypted.
Fixed group in-thread notifications
Use DB sending type rather than in-memory hashtable.
Don't check for auto-initiation on group threads.
Replace LinkedHashMap with a SoftReference LRUCache. Add Slide cache.
Filter MMS weirdness