certify 6.x-2.0

Changes from 1.x branch:

Full rewrite which let other modules expose certificate conditions. Quiz and Book is bundled out of the box (certify_quiz and certify_book).
When upgrading, please run update.php before enabling certify_book, in order to keep the log of which books your users has read.

Included:

#1094774: Can we adapt to include arbitrary content types in certificate pre-requisites?
#942436: Restructure the code for expandability
#1169876: Integrate with book but don't make it a requirement
#1340718: wrong permission for My Certificate
#1400986: Multiple-certificate blocks comes up empty
#1400980: Some conditional blocks show all certificates
#942354: Integration with Organic Groups