Rewrote PHP coding standard w/r/t PSR-2

Submitting this for debate. The idea is that we should adopt some other well-defined coding standard (this uses PSR-2) as a baseline, documenting only our differences.

One inconsistency I've noticed is property names: lowerCamel vs. under_score. In my PR I recommend under_score because most public API properties follow that.

Eventually I'd like Elgg to shift towards the default styles used in PhpStorm and Netbeans.