Added the password/client_secret encryption using bcrypt

I added the password and client_secret encryption using bcrypt. I created an AbstractStorage class to manage the bcrypt component. I implemented a checkCredential() method to check password for backward compatibility (plain text, SHA1 and bcrypt). I added the bcrypt protection in Pdo, Memory (only in construct), Mongo, Redis storage adapters.
I used the Zend\Crypt\Password\Bcrypt component of ZF2. The ZF2 requires PHP 5.3.3+, I changed the composer.json (this is a BC, the previous requirement was PHP 5.2.0+).