load google maps script dynamically so it doesnt need to be included in head manually

The README seems to indicate that the Google Maps API script is taken care of but in the index it was added in there in addition to the google-maps polymer code import.  So when I tried to follow the instructions the map didn't load.  I think it is more convenient if the Googe Maps API is loaded automatically without needing to add the script to head, i.e. as in the instructions in the README.