Added tests

Title pretty much says it all. Tests, tests and more tests.

About 80% coverage on the `eZ/Publish/` folder.

Tests are both unit (in `eZ/Publish/QueryBuilder/Tests`) and API (in `eZ/Publish/API/QueryBuilder/Tests`). API tests execute the builder directly, with all operators for each criterion, and assert that the created query is what was expected.

Broken / unimplemented features are marked as skipped/incomplete, with a relevant comment, so that they can be fixed later.