Changes to image plots to allow speedups for large images

This adds code which allows:
  * colourmaps to be applied only to currently visible region of an image plot
  * downsampling of images
These features are optional, default behaviour is unchanged.

The trade-off is increased computation on every change, vs slower updates and
higher memory usage when the cache does change for the current code.  This is
intended to be used when Chaco is dealing with large image plots which may be
rapidly changing in reaction to user interaction (eg. selecting a region of
an image plot).
Remove downsampling hooks from ImagePlot - will worry about that separately.
Minor documentation fixes for image plots.
Merge branch 'master' into lazy-image-plot

Conflicts:
	chaco/cmap_image_plot.py
Add alternative uint8 mapper to AbstractColormap

Provide implementation via speedups as well.
Fix transform colormap.
Remove a couple of spurious imports.
Remove commented out lines.
Commit updated _cython_speedups.c, since we want that available.
Turn on C division in map_colors() to speed up the loop.
Merge pull request #123 from enthought/lazy-image-plot

Fast and lazy colour maps