Allow plugins to muck with the root node

This exposes total control over the cherrypy mounted tree to plugins.
This will be useful for plugins that want to serve their own custom
application out of the root, and optionally also serve the girder
app out of a subdirectory. ping @mgrauer
Document cherrypy root fiddling
Add automated test for root fiddling
Fix failing python style test
Merge pull request #506 from girder/support-cherrypy-root-fiddling

Support cherrypy root fiddling