Changed contingency computation and its output for continuous features.
Adapted EntropyMDL to use the new contingencies.
Re-added interface to check contingencies for single class values.
Adapted SQL contingencies.
Changed heatmap rendering.

It is now rendered using a sequence of drawing primitives instead of a
large pixmap.

Also implemented adaptive contingency resampling.
Speedup heatmap painting.

(bound the drawing to the exposed area only)
Added heatmap widget icon.
Removed unused code.
Fixed a ValueError in Orange/widgets/utils/scaling.py

Due to the seed for numpy.random being to large.
Painting code cleanup.
Updated data.storage documentation.
Moved the sharpen_region function out the OWHeatmap class.
Implemented recursive heatmap sharpening.
Re-enabled ordering by chi square.
Disable the OSX font fix if using Qt 4.8.6 or later.

Qt 4.8.6 already contains a fix for this.
Explicitly remove items from layout before repositioning them.

Fixes a warning from Qt.
Merge pull request #103 from ales-erjavec/heatmap

Heatmap
Move optional requirements to separate files.
Merge pull request #100 from markotoplak/contingency

Different computation of contingencies
Use itertools.product instead of custom code.