Add blowfish password hashing as an option.

I've added blowfish password hashing to MyBB. It's a bit of a rough implementation, as I wanted to impose minimal changes. I may create a better implementation in the feature branch in the future, but for now I thought "let's share this" and enable forum owners to have a bit safer database.

I've used Anthone Ferrara's password_compat. This library contains password hashing functions which will be in PHP 5.5.