Add LaTeX plugin

This plugin adds a button that lets you write LaTeX and see a preview. It then inserts it with <code class="latex"></code>.

We could use the Desmos wysi editor in the future. And yes, we could probably also make it so that we actually see the rendered TeX in edit mode inside the area, but that will be a bit more work, and I think this is useful to start with.

Inserting new LaTeX:
![insertnew](https://f.cloud.github.com/assets/297042/1958662/569b00ce-821e-11e3-8e42-292dfb74ee46.gif)

Replacing an existing LaTeX:
![replace](https://f.cloud.github.com/assets/297042/1958665/5fa44888-821e-11e3-95cd-bfc0b3f2b700.gif)