Add example of pulling image from threeD view

Re-use the texture WebGL example for this.  Still needs major
cleanup, but it works for testing.
Remove dependence of PIL - use VTK to compress png instead.

This change seems to speed things up, but also allows the extension
to work with any slicer build with no extra dependencies.
Merge branch 'master' of github.com:pieper/SlicerWeb
Improve the documentation a bit