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.
maybe something like this (inspired by  http://github.com/scala/scala-module-dependency-sample):
```
) ++ mimaDefaultSettings ++ (CrossVersion.partialVersion(scalaVersion.value) match {
      case Some((2, scalaMajor)) if scalaMajor >= 10 =>
         instrumentSettings
      case _ =>
        Nil //do not include scoverage
    })
```