Add Distance Matrix support

This PR finishes the work started by @tyler-sommer in #19. Your code was really good! I would love to get more contributions like yours! :)

I don't change lot of things, just fix some CS add doc & add more tests...

For the issues, I have moved `Distance`, `Duration`, `TravelMode` & `UnitSystem` in the `Ivory\GoogleMap\Services\Base` namespace which can be reused accros services.

One more time, :+1: for your work!

Closes #19