Rearrange decrypt API.

1) Change SessionBuilder to only establish sessions via
   KeyExchangeMessage and PreKeyBundles.

2) Change SessionCipher to decrypt either WhisperMessage
   or PreKeyWhisperMessage items, automatically building
   a session for the latter.

3) Change SessionCipher to tear down new sessions built
   with PreKeyWhisperMessages if the embedded WhsiperMessage
   fails to decrypt.
Update SessionCipher javadocs.
Update tests for new API.
Make sure "previous counter" is never negative.
Fix padding problem.
Rename axolotl terminology.

1) ephemeralKey -> ratchetKey

2) Have the caller specify Alice/Bob orientation.

3) Reorganize verification tag.

4) Remove verification tag from key exchange messages, replace
   with signatures in both directions.
Remove verification tag.

1) Remove verification tag from PreKeyWhisperMessage.

2) Include sender and recipient identity keys in the MAC of
   each WhisperMessage.
Update to latest ref10-extract ed25519
Make PreKeyWhisperMessage decrypt more reliably atomic.
Minor refactoring and renaming.
Remove 1 mod 8.
omg trevor
Updated to latest of Trevor's ref10-extract