[Patch] Allow optional passwords in ActiveModel::SecurePassword

The ActiveModel::SecurePassword module is outstanding but I think allowing a password to be optional would allow many more use cases for this feature. E.g. an application using Omniauth in tandem with local authentication probably doesn't require a password for each User. The patch attached adds a :required configuration parameter (defaults to true) and ensures that the authenticate method returns false for a blank password.

This is my first patch submission so please forgive any faux pas I may have made. :)