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)

We may also want to disable the text input box when the keypad is shown, otherwise it gets focus and then the OS-level keyboard may also pop up. I think @wangguan59's version had done something where he swapped out the input field for a div where the typed numbers appeared.