Added support for custom css

Hey Tom,

Not sure if you would like to incorporate this feature or not, so no offense if you don't or if you'd prefer to do it differently. My main motivation for this was to add the on-hover edit/view/etc links that Nico suggested. It adds the custom css directly in the head, which I think is probably ok in this case.

Sorry for all the file mode changes and whitespace changes and some other diffs that don't even look like anything has changed - none of these are things I actually changed. The only differences are the addition of the extra field in the config settings and the echo of this field in the head of default.php 

![screen shot 2014-08-22 at 11 42 59 pm](https://cloud.githubusercontent.com/assets/871211/4019871/cbe6d932-2a90-11e4-921e-ea0dc782b969.png)