Support code coverage using JaCoCo and Coveralls

The Gradle build now supports generating code coverage reports using
JaCoCo. The reports can be run with:

    ./gradle jacocoTestReport

and then view the HTML output at:

    build/reports/jacoco/test/html/index.html

The bitsquare repository has now also been registered with a (free)
account at http://coveralls.io. Our Travis CI configuration has been
updated to publish JaCoCo XML test report data to coveralls.io, which
results in the reports found at:

 - https://coveralls.io/r/bitsquare/bitsquare

For more information on the JaCoCo and Coveralls Gradle plugins, see:

 - http://www.gradle.org/docs/current/userguide/jacoco_plugin.html
 - https://github.com/kt3k/coveralls-gradle-plugin