Documents CRUD

- Added Documents MVC as described in https://www.pivotaltracker.com/story/show/63372400
- Changed routes, nesting Documents within Projects (see ```rake routes```)
- Refactored Projects/Index as described in https://www.pivotaltracker.com/story/show/63372802
- Added migration to Projects changing the ```body```-attribute from ```:string``` to ```:text```