support markdown in title, description, text, and help_text

[yoon]

This issue consolidates #201, #282, #284, #285, #404, #406.
#201: We need a way to provide styling for single words in a question that will work with surveyor iOS. We need to be able to bold, underline, and italicize text.
#282: Subjects should be able to complete a question with images as "questions" and answers. Imagine a puzzle with a piece missing, and we want to test subject's cognitive abilities. Supporting images in both questions and answers allows us to show a puzzle with a piece missing, for example, and the subject chooses the correct piece from a radio list. Ideally, the answers would still support text labels in addition to the images. Less importantly, the images should be renderable in an arbitrary location with regard to their radio button and label: top, bottom, left, right.
#284: support <b>, <i>, <u> tags
#285: @rsutphin I like markdown. There are lots of ruby implementations; I usually use rdiscount. Quick DDGing finds several C implementations:

• Discount is what rdiscount wraps.
• Sundown is used by GitHub and advertises the capability to do non-HTML output.

@rsutphin One other benefit of Markdown is that it is designed to mix its markup with plain HTML. It should be fairly safe to start applying it to existing questions (etc.), even if they are using HTML.

@yipdw Another benefit of Markdown that's pretty huge from a user perspective: Markdown text is (mostly) readable as plain text.

I say "mostly" because some Markdown constructs like footnotes and inline images can make documents look weird, but used judiciously I don't think they affect readability too badly.
#404: <strong> and <br /> tags
#406: the html_safe changes were made in Rails 2.3.8 (and later), as well as the Rails 3.x series.

[rsutphin]

Sundown moved.