Fix showCenterMarker when the map isn't defined with a center.

panTo() doesn't set the center immediately which results in:
1. showCenterMarker not knowing where the center is.
2. fitToMarkers setCenter is overridden by the panTo completion.
Model the centerMarker using google-map-marker.

This allows you to use both showCenterMarker and fitToMarkers on the same map. The center marker will be placed before fit markers runs.
Merge pull request #35 from bamnet/master

Fix updateCenter on first centering, close #32.