Improve the layout of the Constants help panel

Switches to using a table.widefat instead of a definition list and loses the awkward show example on hover. Also now includes a line showing what the Constant has been set to if it's been set.

![screen shot 2014-02-18 at 13 35 25](https://f.cloud.github.com/assets/308507/2195596/fc562a7c-98a1-11e3-9dc8-e9f85754e0b5.png)