Update library configuration

### Git
* add the `composer.lock` to `.gitignore`

### Composer
* add  `codesniffer` with version 2.0.*@dev (while waiting for 2.0 stable) to composer
* update `PHPUnit` to minimum version 4.0
* remove `coverall`

### Travis CI
* move `coverall.io` config from composer to Travis CI configuration file
* add `PHP 5.6` and `HHVM` to Travis CI configuration file

### PHPUnit
* add code coverage to `PHPUnit` config file

The code is now PSR-2 compliant