Code coverage tracking with Coveralls

I've added a maven plugin to send coverage reports to [Coveralls](https://coveralls.io/). The aggregated report is created with Cobertura in Travis for every successful build.

I've added my fork to Travis and Coveralls so you can see how it all bundles together:
https://travis-ci.org/trautonen/handlebars.java
https://coveralls.io/r/trautonen/handlebars.java

You can see the plugin output from the full log at Travis:
https://s3.amazonaws.com/archive.travis-ci.org/jobs/9412789/log.txt

Coveralls has nice badge similar to Travis that I've added to README. If you want to use Coveralls and the plugin, you only need to log in to Coveralls with GitHub account and enable the repository and then change the badge to correct url.

At the moment Coveralls is busy creating the reports, so currently the actual numbers from my fork are not visible, but will be sooner or later.

For more information about the coveralls maven plugin, see https://github.com/trautonen/coveralls-maven-plugin.