Heatmap traitlets now compatible with IPython 2.3.

The change in the traitlets API between IPython 2.3 and 3 is a real pain.
Change max_intensity and radius to floats in heatmap.

... and tidied up. I think users naturally expect these to be floats.
Merge pull request #31 from pbugnion/traitlets_compat

Traitlets compatibility with IPython 2.3