Fixing static_url issue, making package use MarkdownFields instead of Textfields.

Fixing issue in #39 and some other changes.

Fixing issue with static_url setting overriding the user's since it relied on an or that would eval

to the user's media_url. Also modified the ModelAdmins to only override the MarkdownFields instead of
Textfields so that Textfields display normally unless users specify it as MarkdownField's.