use Point whenever possible

Hi again Kelly

I've made 2 changes. Please give them a review.

1. use Point's lat/lng whenever possible
I keep Point's Lat() and Lng() for now. but if you think it's a good idea to make lat/lng visible (Lat/Lng), we can drop the 2 getters later.

2. use Point value (instead of 2 float64 values) in 3 geocoders
I think it makes method signature simpler and takes advantage of zero-valued Point{}.

All changes are formatted and tested.

Cheers
Jerry
It seems like the method signature and the return values have changed here; I think the name of the method should probably also change as well to illustrate this.  Maybe `unmarshalPointFromResponse` or something along those lines.

Additionally, any reason not to return a pointer to a new `Point`?  That way the caller can get a `nil` Point back if there was nothing to decode from the message.