Move google_prettify.

Moving google_prettify into a subdirectory to allow for a unified syntax
highlighting widget.
Made prettify cdn configurable.
Made prettify cdn optional.

In order to make previews available when not connected, a local version
of prettify.js is included and cdn was made optional.
Made widgets compiler respect model excludes.
Merge pull request #156 from caspervonb/syntax-widget

Widgets Overhaul