Refactoring/testing

Refactoring the testsuite in order to cover all sniffs in a simplified fashion across all supported php environments.

- [x] prepare branch for unit testing (#20)
- [x] review phpcs integration
- [x] directly use phpcs classes for unit tests (quick approach)
- [x] abstract sniff properties
- [ ] drop actual literals from fixtures (cleanup)
- [x] review test suite
- [ ] document approach for developers
- [x] TravisCI validates built