Issue #330. Gets the parent of a marker in general, visible or not. Changed README to include it.

As my issue mentioned, it would be nice to get the parent of a marker without the requirement of it being visible. This is what I implemented.

If you have a marker in your MarkerClusterGroup and you want to get the parent of it (Either itself or a cluster it is contained in).
```
var parent  = markerClusterGroup.getParent(myMarker);
console.log(parent.getLatLng());
`````