Make Ahoy.geocode option work for all stores (addresses #50)

This PR addresses issue #50. 

I noticed that this gem doesn't have any tests - so following the example of your searchkick gem, I've added a test harness using Minitest, and wrote a simple test for the VisitProperties#keys method to assert that it does the right thing. I've introduced that as a separate commit in this PR, in case you want to do something else with your testing.