Used the Bar chart as a starting point for heatmap.
plumbing for multi-range/axis
no longer need to pass ranges to glyphs
Completed a simple HeatMap implementation. Also added examples.
working twin axis example
Made the color mapper more generic.
Now supporting palettes.
fix up add_glyph call missed earlier
update tests
Avoid grids in some categorical charts.
Removed checkpoint folder.
fix copy-pasta update error
Change names and fixed examples.
fix typo
respect changes to ranges
another typo
Added HoverTool.
Merge master-in, fixed conflicts.
compute offsets when axes are on the same side
Merge master-in again, fixed conflicts.