Support for multi-device.

1) In addition to the Recipient interface, there is now
   RecipientDevice.  A Recipient can have multiple corresponding
   RecipientDevices.  All addressing is done to a Recipient, but
   crypto sessions and transport delivery are done to
   RecipientDevice.

2) The Push transport handles the discovery and session setup
   of additional Recipient devices.

3) Some internal rejiggering of Groups.
Merge branch 'groups' of github.com:WhisperSystems/TextSecure into groups