Bug 842592: Adding Bangla Input Method for B2G

I have added two Input Methods for Bangla script. The Avro Phonetic Bangla input method and the Probhat keyboard layout. Avro is smart input method that transliterates latin to Bangla, and Probhat is a traditional fixed keyboard layout. Both of these input methods are popular among mobile phone and computer users speaking Bangla.

The keyboards have been thoroughly tested :)

![untitled window_006](https://f.cloud.github.com/assets/932949/1608895/31ceeb52-551e-11e3-89b4-dee84f1dca6d.png)
![screenshot from 2013-11-24 21 34 09](https://f.cloud.github.com/assets/932949/1608896/31fd1d06-551e-11e3-97f4-bfb882af860a.png)
![screenshot from 2013-11-24 21 33 33](https://f.cloud.github.com/assets/932949/1608897/3272afb2-551e-11e3-84fa-963a7664dc82.png)