added scoverage and coveralls support for #58

I've added [scoverage](https://github.com/scoverage/scalac-scoverage-plugin) to the build which can be used locally by running `sbt clean scoverage:test`. Right now its bare bones. It will just generate a report and because of a bug will not even do highlighting in reports.

I've also added coveralls support to get actual highlighted reports. You can see an example coverage report from my branch at https://coveralls.io/builds/1100597.

Whenever a travis-ci build runs, a coverage report will automatically be generated.