Scrypt wrapper: fixed imminent PHP parse error, as function keyword miss...

...ing. Entire strlen method filled with redundant checks. Later in the class its being referenced statically, instead of using the self keyword, which is the proper use case for this context. Also, restored strlen against hash in check method, as it was removed without reason.

Fixes issues listed in #25 https://github.com/DomBlack/php-scrypt/issues/25