STY: Refactor string formatting/templating
Clean-up and move scatter_demo
Cleanup and move fill_demo
Cleanup and move pie_demo
Cleanup and move errorbar_demo.

Split demo in to two different examples
Cleanup and move fill_demo2

Rename fill_demo2 to fill_demo_features to clarify purpose
Cleanup and move histogram_demo
Cleanup and move hinton_demo
Cleanup and move image_demo3.py

* Rename to imshow_demo
* Add smaller version of lena image saved as a numpy binary so that use doesn't require PIL
DOC: Minor rewording
Fix doc build to search in new example sections.

Add new sphinx config variable `mpl_example_sections` and use this list in both `gen_gallery.py` and `gen_rst.py`.
Cleanup and move subplot_demo
Update example section titles
Cleanup and move unicode_demo
Consolidate histogram examples
Cleanup and move vertical_ticklabels demo
Cleanup and move clippath_demo
Rename imshow_demo to image_demo
Cleanup and move polar_bar demo
Cleanup and move polar scatter demo