Add Trumbowyg form type

This pull request enables "trumbowyg" as a form type for rest_in_place.
To enable HTML in the JSON returned from the controller, I had to change line 154 from text() to html(), otherwise the tags would show in the resulting div.

I also had to only enable buttons in Trumbowyg that doesn't unfocus the editor field, since focusout is the event rest_in_place is waiting for to POST.

I would love to enable the link and image buttons also, as well as fullscreen. Suggestions are welcome.

To use rest_in_place with Trumbowyg in a Rails app, I am working at a Gem at https://github.com/ringe/Trumbowyg/