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).
Fix PDF clipping and add an add_page() method.
Don't initialize  a new page until render_component() is called.
Support landscape page orientation for PDF output
Merge pull request #115 from jilott/feature/landscape-page-map

Support landscape page orientation for PDF output
Merge remote-tracking branch 'origin/master' into feature/multipage-pdf
Overlay should force a border to be drawn.

Added border_visible trait as condition for drawing the border. Default
is to draw the border to maintain previous behavior.
DOC Remove reference to old 'enthought' namespace.
Merge pull request #118 from enthought/doc-remove-enthought

DOC Remove reference to old 'enthought' namespace.
ENH: Expose the text color in the TextBoxOverlay.
Merge pull request #120 from enthought/feature-text-box-overlay-text-color

ENH: Expose the text color in the TextBoxOverlay.
Make LineSegmentTool use x and y mappers so it works with BasePlot2D instances.

This shouldn't affect BasePlotXY usage.
Add an 'update' method to AbstractPlotData.  General clean-up of ArrayPlotData.
And actually get the code to work.
Merge pull request #121 from enthought/fixes/xy_mappers

Make LineSegmentTool work with BasePlot2D
Note changes to API.
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.