Add PSR-2 to "Our Definition of Quality"

This adds PSR-2 into the "Our Definition Of Quality" list (as per https://twitter.com/philsturgeon/status/509390953724272640) so as to be explicit about the supported standards.

I've tried to group the PSR-related items together, though I've added it below PSR-4 as I feel PSR-4 is slightly more noteworthy than PSR-2. Feel free to muck with wording and placement.