Mercury markdown

Allowing to edit documents in HTML or Markdown within Mercury editor
PT story: https://www.pivotaltracker.com/story/show/72843890
Demo at: http://yaro.herokuapp.com/editor/projects/quae-ut-eos/documents/temporibus-necessitatibus-iste

Features:
 - can switch from Markdown to HTML for existing documents, preserving the formatting
 - can switch from HTML to Markdown, converting to plain text

Changes:
- mercury function to o the switch
- Documents Show view - conditional to render html or markdown
- Documents controller to save the chosen format
- format: string field to Document model

Minor changes:
- spec refactorings in documents_controller_spec and show_spec