Add needsRehash function

[pinoy4]

I believe it would be very convenient to have a function that evaluates whether a password's hash has been generated using lower costs that the current implemented ones. I like to store my costs in a configuration file so that if I am to upgrade my machine's hardware I can increase the cost easily, this way maintaining my new hashes updated with the latest machine power. I also like to update old hashes when they were generated with lower costs so if a password is verified I'll go through this check. I already have implemented this function in my local scrypt.php file and if you agree this is a good idea I would like it to be implemented on your repo.

[beyonderyue]

Hi,
Can you submit your PRs to https://github.com/beyonderyue/php-scrypt ?
Thank you.

[pinoy4]

Hi, I'm not really sure this is very relevant to php anymore since the added crypto functionalities in newer php versions. If you insist I could still take a look at what I did back then and make a PR

[beyonderyue]

@pinoy4 when you submit this PR, you can tell me the PHP version.
Thank you.