Generate code coverage reports and publish them to coveralls

This PR adds code coverage generation to eZ Publish LS.

The code coverage report is generated during the Travis-CI build and then published to coveralls.io.

**A free account has to be created at [coveralls.io](http://coveralls.io) by eZ Systems and this repository has to be added there first!**

I have enabled it for the master branch in my repository, so you can see the result here:
- Overview: https://coveralls.io/r/jeromegamez/ezpublish
- Build report: https://coveralls.io/builds/276703
- Coverage Report Badge: https://github.com/jeromegamez/ezpublish/blob/master/README.md

The build on my master branch is marked as failed because of IO errors on the Travis servers, this is not related to the changes.

Due to eZ Publish's history, it is understandable that the code coverage is not as high as we would like, but I think and hope that seeing the badge and reports could make us strive for more and extensive tests. I know *I* would be even more motivated. I hope, you like the idea.

Cheers
:octocat: Jérôme