Add support for PHP, C# and Erlang in fenced code blocks

This fixes https://github.com/atom/markdown-preview/issues/75, fixes https://github.com/atom/markdown-preview/issues/79, and fixes https://github.com/atom/language-gfm/issues/23. 

BEFORE 

![screen shot 2014-05-10 at 6 53 53 pm](https://cloud.githubusercontent.com/assets/38924/2936700/1a8f93a2-d86c-11e3-9a40-1de3d90e0ba7.png)

AFTER

![screen shot 2014-05-10 at 3 59 46 pm](https://cloud.githubusercontent.com/assets/38924/2936696/da2e088e-d86b-11e3-9b70-8a9869b4e98d.png)

Note: this pull request adds highlighting for fenced code blocks in the editor only, not in the Markdown preview. I've made [a separate pull request for that](https://github.com/atom/markdown-preview/pull/80). Also, notice that PHP had support in Markdown preview, but not in language-gfm.