Prevent an error with np.digitize and 0 len input.
Refactored 'Heatmap' widget.
Changed how children array is tested for valid objects.

numpy.where yields different results with numpy v1.9 and v1.8
for object arrays containing 'None'.
Fixed update time computation.
Merge pull request #101 from ales-erjavec/heatmap

Heatmap refactor