0.12

Primary changes:

• Updated the statistics for timers when using the servo registry to include totalTimeSquared. Precomputing this gives us a convenient way to determine standard deviation across groups on backends that allow aggregation.
• Remove listener methods from Registry interface. For most of the interesting use-cases it was insufficient and a dedicated Registry implementation can do a better job of generating a meaningful stream of events.

A comprehensive list of changes can be found in the commit log: 0.11...0.12