Generate code coverage reports and publish them to coveralls

This PR adds code coverage generation to the eZ Publish Kernel.

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 before merging the PR. When you have created the account (provided you want to use the code coverage reports), I can also retrigger a build to check if everything works.

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-kernel
- Build report: https://coveralls.io/builds/278099
- Coverage Report Badge: https://github.com/jeromegamez/ezpublish-kernel/blob/master/README.md

Cheers
:octocat: Jérôme