added HMAC support to PassHash class FS#2794
use HMAC in media_token FS#2794
use hmac for external ressource hash FS#2794
use HMAC for CSRF security tokens FS#2794
use HMAC in password reset token FS#2794
make password reset token completely random

No need for HMAC here because there's no length attack vector here. We
only care for the existance of the file and each reset request is
completely (random) independent from each other.
Merge pull request #226 from splitbrain/hmac

Implement HMAC for signature tokens