Markdown madness

Adds the Redcarpet markdown parser. Redcarpet loads a parser from a the
load_parser method in ApplicationController. Controllers call it in a
before_filter on any views that could display text. (Would like to determine
if it's possible to keep the parser initialized constantly.)

Item#get_content requires the parser to be passed into it in order for it
to call render. It does this so that it can internally escape any html before
sending the markdown to the parser. (The output has to be marked html_safe.)

Parsed text appears in an item's show page, as well as any previews or any
previews or thumbnails of items. The css selectors .text-content, .item-preview,
and .item-thumbnail have their own styles. The content can have h1-h3 in it.