Markdown in MongoMapper

If I create a Markdown type for MongoMapper, you'll do something like this:

```ruby
class Post
  include MongoMapper::Document

  key :text, Markdown
end

p = Post.create(:text => "_Foo to the bar to the **foo**._")

p.text # => "_Foo to the bar to the **foo**._"
p.text.html # => "<em>Foo to the bar to the <strong>foo</strong>.</em>"
```

Eventually, this can move to [eturk/mm-markdown](http://github.com/eturk/mm-markdown).