[4.1] Added injectable CSPRSG

Added injectable cryptographically secure pseudo-random string generators and improved on the existing CSPRSG logic.

Complete with [docs](https://github.com/SammyK/facebook-php-sdk-v4/blob/injectable-prsg/docs/PseudoRandomStringGeneratorInterface.fbmd)! :)

This addresses issues from #262 & #128 and crosses off another #136 TODO. :)
:+1: