Fixed bug with default point_radius and max_intensity.

Using the default values caused a bug. Now fixed.
Fixed heatmap defaults for IPython v2.3.
Refactored IPython 2.3 compatibility into stand-alone module.

Created an ipy23_compat module to abstract away the compatibility
issues between IPython 2.3 and IPython 3.
Merge pull request #32 from pbugnion/heatmap_defaults

Fixed bug that arose when using heatmap with default parameters.