Updated keyboard layout with additional buttons - Clear, Equals and Hide keyboard

Keyboard layout changed and extended with new buttons:
[C] Clear - clears value of textfield
[=] Equals - evaluates current expression in textfield
[Hide keyboard] - evaluates expression and hides keyboard.
![new-layout](https://cloud.githubusercontent.com/assets/747340/3074663/40a08782-e34c-11e3-8b80-b7f6d7c60385.png)