add software keyboard support to iOS

Code is not mine, taken from:
http://www.badlogicgames.com/forum/viewtopic.php?f=17&t=11788

and reformatted

it works nicely though:
![ios_keyboard](https://cloud.githubusercontent.com/assets/1393288/2575690/19d7f5a8-b959-11e3-8564-0001ef47bf43.jpg)