Consistent footnote number width.

Fixes #499 


I'm not 100% sure I put this in the right spot, but it does fix the issue. Before / After screenshots below. Edited the html to show that there's plenty of room for larger numbers.

## Before
![screen shot 2014-11-27 at 6 16 35 pm](https://cloud.githubusercontent.com/assets/3853/5220157/8ebbc046-7661-11e4-9145-4267c23297ec.png)

## After
![screen shot 2014-11-27 at 6 16 19 pm](https://cloud.githubusercontent.com/assets/3853/5220158/8ec3813c-7661-11e4-86f6-bda261ad149a.png)