Adding version control to documents.

https://www.pivotaltracker.com/s/projects/982890/stories/63371936

. added gem paperclip
. added relationship to document model
. implemented code to `show.html.erb_spec.rb`
. implemented code to `document_spec.rb`
. implemented code in views to show document's history
. created scenarios in cucumber to test revisions for documents
. cukes and rspec passing