Implementing Quickreply

I've implemented the feature, right now the way it works if TextSecure is locked is it will queue the sending of the message until it is unlocked.
However, if the service is killed before that the message will be lost. We can avoid this using START_REDELIVER_INTENT but this gives some slight chance of resending messages if the service is killed between the sending of the message and the stopSelf call. I opted to avoid this for now.

I tried adhering to your code guidelines but this is my first look into this project, and frankly I'm not a major Android developer though I think I did alright.

I avoided duplicating the "run tasks when you get the master secret" logic in SendReceiveService by extracting it a parent class "MasterSecretTaskService".

Let me know what you think about the way I implemented this.

Issue #299