Adds functionality to render Markdown file attachments in Review Board.

The rendering is handled by the "markdown" Python package which, given a file, returns the HTML code for it. This HTML code is then passed to the Backbone view for the Markdown Pluggable UI through the template and rendered in the browser.

Note: This code depends on this review request: http://reviews.reviewboard.org/r/3613/

"Valid" Case:
1) Created a review request and uploaded multiple random files to it
2) Then, uploaded a valid Markdown file (with the extension .md) to that review request
3) Clicked the "Review" link for the Markdown file
4) The new page loaded should render the Markdown file

"Invalid" Case:
1) Created a review request and uploaded multiple random files to it
2) Then, uploaded an image to that review request by changed the file extension to be ".md". For example, wallpaper.png -> wallpaper.md
3) Clicked the "Review" link for the invalid Markdown file
4) The new page loaded did not render anything (i.e. it displayed the title and caption but nothing was displayed in the section where the HTML would have rendered)