Add can_impersonate? functionality

Per the comment I just left in the Issues list, here is the functionality to add the can_impersonate? method option to the gem.

See the changes in the readme.md to see the specifics for usage, but its dead simple.  I tried to mirror exactly what you were doing for the is_staff? method.

Makes the gem a lot more useful for complex authentication / role cases.

Let me know if you have any questions or want any changes.