This adds support for a more expressive (albeit more verbose) IdentMap
structure. It makes it easier to annotate the structure with additional
data (such as names to associate with the IDs), to assist with
maintaining the IdentMap.
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