Fixes to the internal API hooks for the sbt server.

* Alter the TaskProgress listener key to be `State => TaskProgress` so it
  can be instantiated from the current server/sbt state.
* Expose the xsbti.Reporter interface for compilation through to sbt builds.
Merge pull request #1082 from jsuereth/wip/sbt-server-api-fixes

Fixes to the internal API hooks for the sbt server.