Create a more polymer-friendly way of styling map-markers

[phidias51]

It would be useful if there was a way to style the map markers. Something along the lines of:

google-map-marker#my-location{
  color: blue;
}

google-map-marker[selected]{
   color: green;
}

Also, the user should be able to set the icon for a map marker with a simple icon reference, like this:

<google-map-marker icon="maps:person-pin">...