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.
Why the eval()? To get a copy?

I think this entire operation can be simplified by passing the entire AVP-object to the remove_avp-function and using indexOf to find the correct index in the AVP list. See for example https://github.com/SpriteLink/NIPAP/blob/master/nipap-www/nipapwww/public/controllers.js#L36.