Add an 'update' method to AbstractPlotData.  General clean-up of ArrayPlotData.
And actually get the code to work.
Note changes to API.
Fix event emission on delete (thanks to @MatthieuDartiailh for the report)
Improve signature of update methods.
Fix mis-named variable.
Update scalar_image_function_inspector to use new Update_data() method.
Merge pull request #124 from enthought/feature/plot_improvements

ArrayPlotData Improvements