Add time to messages when they are within the week

//FREEBIE

I had many friends requesting this, and it could be a nice addition to this pull request also. 

https://github.com/WhisperSystems/TextSecure/pull/1352

https://github.com/WhisperSystems/TextSecure/commit/f63cf004703f456f90383f202b42725937b2bab3