Add maxZoomOnClick option

Added maxZoomOnClick option as the maxZoom option affects the click
action of the clusters. If you click on a cluster pin and the zoom level
exceeds the maxZoom, the zoom is reset.
Cluster image path optional

Make image path optional. If falsy or empty string the img tag is not
added.
Ensure cluster click causes an increase in zoom

When setting Google Maps to show a boundary it tries to maintain spacing
between that boundary and the edge of the container. This can be a pain
on small screens because we're short on screen space already. we need to
store the maps current zoom level and then test it once the boundary has
been set. If the zoom level has not been changed then increase the zoom
level manually