TinyMCE for projects

Related to Pivotal Tracker story *Mercury Editor for projects description* https://www.pivotaltracker.com/story/show/64507678.

[TinyMCE editor](http://www.tinymce.com/) for project description in new/edit form, using [tinymce-rails gem](https://github.com/spohlenz/tinymce-rails). My solution for initialization under Turbolinks is probably clumsy - feedback appreciated. It also takes away some of the flexibility of TinyMCE configuration and only allows the default layout specified in config/tinymce.yml.

To see it in action: http://websiteone-nga.herokuapp.com/projects/websiteone/edit.
Since Rails compiles all the assets into one single file and the browser does a excellent job it caching it, Turbolinks does not save us much except a few bytes here and there.