Update backbone.googlemaps.js

Hi..

in sample.html i find and error (suggested by web console).

it seem that default model for GoogleMaps.LocationCollection isn't GoogleMaps.Location;
i resolve adding

model : GoogleMaps.Location, at line 75.. now it works, but i don't know if it's the best way to resolve it.

i hope it helps you.

Bye and thank's for the great job.