Story EZP-21956: integrate coveralls.io

Based on #574 by @jeromegamez.

See https://jira.ez.no/browse/EZP-21956.

Adds configuration for coveralls.io:
- add generation of coverage for a subset of our tests, one report for unit tests, and another one for legacy integration tests.
- uploads those tests to coverall
- adds the coveralls.io badge to README.md.

# TODO
- [x] Refactor to avoid duplicated XML files
- [x] Try running on PHP 5.5 to shorten execution time