Updated the coding standard to PSR-2

I was sort of hesitating to push this PR, because it's not *really* my place to set the coding standard. However, I did observe that the current coding standard seems to be no coding standard at all, so I'm simply suggesting using [PSR-2](http://www.php-fig.org/psr/psr-2/).

There are no actual code changes in here, just CS changes.