Projects description editor

![projects_description_editor](https://cloud.githubusercontent.com/assets/1007981/4351091/cbae3378-4202-11e4-8708-f802d4dc2777.gif)

Pivotal story: https://www.pivotaltracker.com/story/show/64507678

TODO: 
— security. how to safely store user entered HTML into the database? advice appreciated
— controller tests for Mercury methods added to Projects controller.
— fix one Cucumber failure (capybara isn't looking in the right place for flash output)
— i seemed to have broken the home tab somehow.