WIP: Stats endpoint

This WIP branch will fix #152 when completed.

The following should be implemented before the branch is merged with develop:

- [x] The stats access event listener should be able to handle wildcards in the IP addresses in the white-/blacklists. Fixed in 00a3df6 (for IPv4 only).
- [x] The stats model should have a simple `key => value` container that event listeners can use to inject custom statistics. Fixed in a2482d1.
- [x] Support for IPv6 in the `StatsAccess` event listener. Fixed in 4080ee8.

Any other features that should be added?