Adds PHPCS, improves a bunch of things

Description of this PR shall be improved too :laughing: 

Well, this packs:
- Adds `squizlabs/php_codesniffer` for obvious reasons;
- Tells **Travis** to check code-style via `phpcs` rule on *Makefile*;
- `$this` on `return` is really ugly (IMO), using `self` instead (yes, PHPDoc does support both);
- CS/readability improvements.

:beer: :+1: