Adds support for NDarray data in heatmap.

Data passed to heatmap utility function can now be a numpy array.
Updated IPy 3 heatmap example notebook.

Replaced the call array.tolist() by array, since heatmap now supports
numpy arrays.
Updated info on committing notebooks in CONTRIBUTING.

Script given now reads nbformat from notebook. It now works with
notebooks written in IPython v2.3 and IPython v3, regardless of the
IPython version installed on the computer.
Updated heatmap example for IPython v2.3.

Remove use of tolist().
Merge pull request #18 from pbugnion/heatmap_ndarray

Heatmap now acceps numpy arrays.