Because the tool is using utc-timestamps everywhere, this emphasizes
this fact (by default) in the output.
If you don't want timezones behind each datetime in your output, just
remove the "%Z" from the `datetimeformat`.
Fix#9
* use collections.MutableMapping as base
* use one dict to store all types of Clients
* don't use implicit += but explicit .setdefault in tsstats.log for client-adding
* give all clients to sort_clients in tsstats.template
* remove now unneccessary clients-assignment in tsstats.utils.sort_clients