Add Password Expiration feature

This is code to add a submodule for easier Password Expiration, a security concern requiring app users to periodically change their password.

The submodule API adds:
```
# password expiration
require_valid_password # this is a before filter
@user.need_password_changed?
@user.need_password_changed!
@user.update_password!(password_params)
```

Thoughts?