v1.2.0

Quite a few changes to reflect new behavior around identity keys rolled
out in the Signal apps in June 2017:

• We always trust a new identity key when receiving messages
• We have a few requirements for trust when sending messages:
  1. it can't have changed very recently
  2. if the previous identity key had been verified by the user, the
     user needs to explicitly approve a new identity key

Expose ability to delete all existing sessions - for proper reset (#35)

Allow caller to provide fillMessageKeys limit - allowing customization
of the maxiumum number of one-sided messages in a conversation (#29)

Dev:

• A number of changes to make the branch build in CI, add badge, etc.

Note: we appear to have some unreliability during our Firefox 34 builds
on Sauce. A little less than half the time the test fails with an out of
memory error.