Make reference links more consistent and identifiable

This addresses mitsuhiko/flask#923.

Updates the style of reference links to be closer to that of normal links to differentiate them from their non-link counterparts. Here's an example:

![screen shot 2014-02-13 at 11 25 34 am](https://f.cloud.github.com/assets/55005/2162156/e89ca6fe-94ce-11e3-9579-eb083e398296.png)

The reference at the bottom should be a link, but is missing a destination. I'm guessing that's a bug with whatever generates those references.