UI for projects/documents 

Tracker story https://www.pivotaltracker.com/story/show/64162074
And https://www.pivotaltracker.com/story/show/64130846

- Implemented `act_as_tree` gem
- Redesigned Documents Show page
- Added `children` and `roots` methods
- Redesigned documents/_form to use hidden field for `parent_id`
- Added "Create new project" button to sidebar as a plus sign
- Added cukes and specs to cover new functionality