Added IPython 2.3 compatibility.

* Changed reference to WidgetManager.
* Added height and width as CSS variables when IPython is version 2
  (rather than as traitlets).
Minor cleanup of manager compatibility hack in heatmap_view.
Exposed height/width in Heatmap for IPy v2.3.x.

if w is a heatmap, w.height and w.width are now supported for IPython
v2.3 as well as IPython v3.0.
Heatmap rendering height/width correctly for IPython v2.3.

The heatmap view now correctly reads its width and height information
from the model.
Merge pull request #10 from pbugnion/ipy2_compat

IPython 2.3 and IPython3 compatibility