Add expires attribute

This adds a new attribute called 'expires' that can be used to set an expiration date for an assignment of a prefix. It does not include the web-UI parts.. only backend, pynipap and CLI at this point.

The backend accepts an iso8601 date, None is treated as infinity (ie, never expires) and we do fuzzy parsing to allow human dates likes "tomorrow" or "two years" (expiry is in two years from now).