Show state picker if the country selected in the country picker is US

Also hide it again if the user later selects a non-us country.

This is really easy; I've already wired up a method in the controller that is called whenever the users makes a selection in the country picker.

This hardest part will be ensuring that the value for US in the country picker is the same value you are comparing to.

This issue cannot be dealt with until issue #70 is completed.