Add initial support for code coverage checks

DO NOT MERGE, changes default build to 2.10.4.

This shows how to enable scoverage, which integrates with: http://coveralls.io/ via: https://github.com/scoverage/sbt-coveralls

These plugins don't work with 2.9.

Currently:  [info] [scoverage] All done. Coverage was [91.28%]

Not too bad, but it would be nice to get that to 100% soon.