Password expirations

Following patch enhances the password policy module and adds password expiration feature. Admin can set the time period after which user gets blocked if he hasn't changed his password.

Admin can also set the days before the password expiration, on which a password reset reminder will be mailed to user which password is about to expire.

Normal and retroactive behavior can be configured when a new policy is enabled. Normal means that if password expiration is set to x days, then all accounts are valid during x days after enabling the policy and only after x days unchanged passwords expire. Retroactive means that on the password policy enabling all older than x days passwords expire immediately.

System can be configured to behave differently on the password expiration:
a) user is blocked and only admin can unblock him
b) user is sent to change password page and if he won't change his pw, he won't be able to login again (is blocked).