Change documents sections

PT Story: https://www.pivotaltracker.com/story/show/72330554
Heroku app: http://damp-lowlands-2351.herokuapp.com/ (you have to be logged in to seeing the "Change section" button in a document. 

Brief description of the changes: 
- added button for changing the section in the documents show page
- using pop-up window for choosing the new section (as suggested by @YaroSpace here: http://agileventures.org/projects/websiteone/documents/lofi-documents-management) 
- It won't be very clear in heroku's instance because of the funky names, but the sections available for choosing are the ones amongst this project, as well the current document isn't available for choosing. 
- It also doesn't show the children of the current document in order to avoid loopholes. 
Well, I am ok to do it in some other way. Just if... this doesn't get deleted, then we won't be able to use the update method in the documents controller (where we call the function for changing the parent)