Edit feed UI

I've implemented a rather crude way of editing the feed URL from the UI (ref. #317). It appears to work, however there's a couple of issues I'd like to address before merging.
 
* [x] I'd like to use the PUT verb for the update action, but it turned out to be a bit difficult to implement. I looked into using `Rack::MethodOverride` for this, but didn't manage to get it working. Any suggestions on how to implement this would be appreciated.
* [x] I'm not sure I like the way the buttons appear in the feed list. Any input on the styling would be appreciated.

Some screenshots:
![2014-05-26-204851_1366x768_scrot](https://cloud.githubusercontent.com/assets/543455/3085039/c12cebfa-e506-11e3-91d4-baedd8c4d43c.png)
![2014-05-26-204906_1366x768_scrot](https://cloud.githubusercontent.com/assets/543455/3085040/c3ae2934-e506-11e3-9376-29197522567a.png)