Implemented CSV download of AdImpression data.
Adds support for multi-sites.
The small fix
Merge branch 'ticket/22' of https://github.com/melinath/django-adzone
Incorporate sites, add tests, remove user

 - Incorporate a `sites` M2M for AdBase, allowing ads to be shown on
   select sites (starting with branch from @Klice).  Update tests to
   function with sites.
 - Incorporate CSV download of ad impressions, from branch by @melinath.
 - Remove `user` field from Advertiser. Not sure what it is for, and
   it'd just confuse my admins.
 - Change strategy for random ad selection to something more performant.