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
No special reason returning Point value (rather than Point pointer). Just wanted to make it a smooth change from lat=0 lng=0, to Point{} (which is {0, 0}, not a nil anyway). So peace of mind.