Status changes messages in chat window

 and possibility to enable in settings

![status changes](https://cloud.githubusercontent.com/assets/2544251/4600433/60154cc4-50d4-11e4-922e-7c3271e2ebe0.png)
![status settings](https://cloud.githubusercontent.com/assets/2544251/4600432/600afe9a-50d4-11e4-8609-9b0d4c2e1098.png)

also, removed one emit friendStatusChanged signal in Core.cpp, it was emitted twice per one status change.
Created new SIGNAL contactSignIn, it will be used later for sound notification and pop-up widget.