Markdown previews of posts

Preview hide/shows based on value of textarea
The composition textarea resizes based off input

![004f26e59cff1ef6ac4654c3c90bfaaa](https://f.cloud.github.com/assets/239123/1845234/919b8ab0-7579-11e3-8521-a73aa020fcbe.png)

This resolves #99