Allow users to use latitude and longitude to access lat and lng for GeoK...

This pull request provides the ability to call latitude and longitude on Geokit::LatLng objects.  Many existing projects use the more verbose names of latitude and longitude.  For projects that do this you are utilizing two different names for looking up the same objects when using Geokit.  This allows the user the ability to choose whichever naming convention matches their existing code without impacting current users of Geokit by aliasing Geokit::LatLng attr_accessors `:lat` and `:lng` to `:latitude` and `:longitude`. 

I'll be more than happy to change the test methods to test equivalency instead of object reference if you would prefer as well.