Set cell value to null when user presses DELETE or CONTROL+BACKSPACE

NOTE: the consumer's observable will still need to understand and handle the null value. If
you are using a numeric extender, for example, you will need to ensure it can handle null
without changing it into a NaN.
0.0.3
Merge pull request #13 from jstclair/add-keybinding-for-setting-null

Add keybinding for setting null