auth: added method user to the PasswordToken type
auth: implement getPasswordToken function
auth: add ResetPassword function

This function will generate a new password for the user, and mail it.

Related to #200.
auth: added stress test for generatePassword

Related to #200.
auth: unexport passwordToken type
auth: use public variable to signal user not found error
auth: use public variable to indicate authentication failure
auth: add some docs on public errors
Revert "auth: use public variable to indicate authentication failure"

This reverts commit cd343f5e29ec416bf435f2da739fe6e9d665170f.

This is not really what I should be doing. Sorry about that :)

Conflicts:
	auth/user.go
api: use ErrUserNotFound in login handler