Fix for SSHA Dovecot Scheme Problems [Issuse #20]

Another fix for Issue #20, which doesn't allow password change for users, when the secure password hashing scheme SSHA512 (and others) is used.

I tested this fix in my setup with SSHA512. Please review this fix and test it with other schemes. Maybe other Dovecot Scheme could be implemented as this should be very easy. As I lack the time to do and test everything, I just implemented the parts I use in my setups.