Script tidying and new build

related to issue #9

- added a little utility function to traverse and focus on element
sequences
- sprinkle $.traverse and $.attemptFocus into Prospectus. I think (esp
the key code section) reads easier now.
- broke jquery UI :tabbable/:focusable into their own file and added it
to _libraries build
- broke $.toggleAria into its own plugin and added documentation
- updated site build with all of the above