Updated keyboard layout with additional buttons - Clear and Equals (that hides keyboard)

Keyboard layout changed and extended with new buttons:
[C] Clear - clears value of textfield
[=] Equals - evaluate expression in textfield,  and hide keyboard
![new-layout](https://cloud.githubusercontent.com/assets/747340/3076567/74a21eb2-e3ea-11e3-80c4-17eb9739118b.png)