Added sql-change-password plugin

A configurable password-change Plugin for SQL (as proposed in #256). It allows to config the database connection as well as the fields for the user_id, username and password used to update the password.
It uses the php crypt() function to encrypt the password. It also detects if the password in the database has a dovecot "{X-CRYPT}"-prefix and adds it it afterwards to the new password again.