Add Counter Groups for Sorting counted things by count

[sappenin]

If a set of Counters belongs to a given "group" or "tag", then we can create an index of all the counts for the counters in a given group, and query the datastore for ordered lists of counters. In this way, we can enable leaderboard-type activity.

For example, if there are N number of counters tracking follower counters (i.e., one counter per user being followed) then we could add each of these counters to the "follower" group. Next, we could query the counter service for the "most followed" or "least followed" users by getting an ordered list of counters in the "follower" group.

[sappenin]

See issue #3 when implementing this feature. The CounterGroup contains a surrogate field for tracking the counter's counter in an "eventually consistent" manner.

[sappenin]

This is implemented via the commits for issue #18. See issue #19 for the final pieces to enable this feature.