Add prefix avps

This adds AVP support to prefixes in the backend, pynipap and CLI. www-UI is deliberately not addressed as we a) do not currently have a consistent environment, the prefix edit page is in jquery while prefix add is written with Angular and b) I don't understand AngularJS well enough to implement it. Hope @garberg can address that :)
Hm, I think we should specify an exception here, catching any exception IMHO has the bad habit of biting you in the ass :stuck_out_tongue: As long as ```avp``` is a string, I guess the only exception we can get is an ```ValueError```, if the string doesn't contain "=":

```
>>> k, v = "asdf".split("=", 1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: need more than 1 value to unpack
```