Markdownify the changelog

Store the changelog as valid markdown for enhanced readability when viewing on GitHub.com.

## [Old busted](https://github.com/mojombo/jekyll/blob/master/History.txt):

![screen shot 2013-05-06 at 9 53 14 am](https://f.cloud.github.com/assets/282759/466227/64fdf2de-b654-11e2-865f-ea29affbf304.png)

## [New hotness](https://github.com/mojombo/jekyll/blob/master/History.txt):

![screen shot 2013-05-06 at 9 53 27 am](https://f.cloud.github.com/assets/282759/466229/6b2b3c20-b654-11e2-9b47-991990f6098b.png)