Add scoverage test coverage tool

 * Integrated scoverage into our pom.xml using instructions at: https://github.com/scoverage/maven-scoverage-plugin

  After running "mvn test", an HTML coverage report can be generated by running
"mvn scoverage:report". The result will be written to target/scoverage-report.

  scoverage currently does not support line-by-line code coverage reports when
using Scala 2.10.4 (our version). However, we can still get a per-class
coverage report. When we update Scala to either 2.10.5 or 2.11.2, this should
be fixed, according to scoverage/scalac-scoverage-plugin#16

  As part of this, I also ended up reformating pom.xml to use 2-spaces throughout.

 * In addition to the scaladoc, Travis will now push to the gh-pages branch a
"scoverage-report" directory giving our code coverage report.

Closes #212.