[new] Test against PHP 5.6 without allowed failure
[fix] Fix return by reference error in Enumeration::getType()

Probably only affected PHP >=5.6
[dev] Switch test framework from Codeception to PHPUnit
[new] Use the 'clean' template for API docs
[dev] Reorganise code files
[dev] Update Travis test tasks
[dev] Delete unneeded config file (cont. 33ae781)
[dev] Update docblocks
[dev] Add Makefile for build and test tasks
[dev] Auto-update gh-pages on tagged commits