Added IPy3 demo notebook of heatmap.
Added heatmap demo for IPython2.3.
Updated CONTRIBUTING guide for IPy notebooks under VC.

Added information on how to commit an IPython notebook under Version
Control.
Updated heatmap_demo on IPy3 to match IPy2 version.

Also updated information in CONTRIBUTING guide on how to commit
notebooks to the repository.
Merge pull request #17 from pbugnion/heatmap_examples

Adds example notebooks for heatmaps.