Secure password hashes

Modifications:

* Added salt
* Added multirounding
* Changed SHA1 to SHA512

I can't save my passwords in plain SHA1 :)


Disadvanteges:
* Depends on mcrypt extension.
* Depends on `hash_pbkdf2()` function. This function appeared in PHP5.5. Solution for PHP5.3 and PHP5.4: "https://github.com/rchouinard/hash_pbkdf2-compat".