www: add AVPs for VRFs

This adds the ability to add, modify and remove AVPs for VRFs.
Implemented using AngularJS - my first Angular code, yay! =)

Fixes #637.
Same as previous comment, ie not my code. I copy-pasted it from somewhere.

Not sure if we should even try to catch errors like this.. I mean, you can't really call this function with a name that doesn't exist. If we are going to catch everything we can dream up, we'd have code size explosion. What do you think?