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. 
I added this if on line 95 above last night, because I though it was nice to double-check. Actually in the document's show view, in the each for the categories I am checking for: 
- if this is the current parent category
- if the category is a child of the current doc so we don't get in a loop, where the both docs are parents of each other
I think the only possible way of breaking this is to interact with the get params by guessing (or seeing somewhere else) current doc id. 
If you prefer, I would put a double check for those in the change_document_parent method itself?