Software keyboard for Android

Fixes #270.

Summary of changes:
* Add a new Backbone View to render an onscreen numpad.
* Only show numpad on Android devices.
* Only show numpad on questions that have single input, numeric responses.

Screenshot:

![image](https://cloud.githubusercontent.com/assets/1680573/3559719/5187904e-094f-11e4-8d3e-af7151c03d6f.png)