Fix view format=date so that it doesn't crash on empty fields
Docs update
Use prototypal inheritance to speed up the inheritance of widget variables

Now we reuse the prototype mechanism to let javascript do the work of
searching up the parent chain.
Tweaks tests

This was the only place we were using the `variables` option to the
widget constructor.
Improve performance of tag handling with some caching
Merge remote-tracking branch 'origin' into etchTheme