Separate all akka-related stuff into it's own module.

All Akka-related instrumentation and code has been moved to the kamon-akka module,
including the filters for actor, dispatcher and router metrics. Also the following
changes are included:

  - Router Metrics are now working properly, related to #139.
  - Cleanup the log output for this module, related to #142.
  - Some minor cleanups in various tests.

This PR breaks the reporting modules which will need to wait for #141 to be ready to come back
to life.