Add statistics according to #248

This add basic statistics for VRFs, pools and prefixes in the backend, exposes it via the API and enables support in pynipap so that the CLI and the web can display statistics.
Not sure, it's not like we add that many new features so that having a wrapper function would considerably ease this. We have a lot of different output formats, like a list or a dict or lists in dicts so the cast-as-string-function would still be quite specific. We have something similar (a shared function to clean up results) in xmlrpc.py, see https://github.com/plajjan/NIPAP/blob/248-add-statistics/tests/xmlrpc.py#L71

I don't really mind the current structure. At some point we are probably going to adapt a JSON REST API anyway and then this will be moot ;)