Fix issues with FeatureLayer and ClusteredFeatureLayer

* Resolves https://github.com/Esri/esri-leaflet/issues/119 (not able to remove `ClusteredFeatureLayer`) and https://github.com/Esri/esri-leaflet/issues/122 (Features disappear at certain zooms).
* Fixes alignment issues with the clustering demo so shapes are aligned with their centers better.
* Allows use of "L.esri.ClusteredFeatureLayer" without passing a Cluster object.
* Rebuilds distribution files.