Beta 4

This gets everything ready for another release with a few new features and some small refactoring. It also cleans up examples a little.

* https://github.com/Esri/esri-leaflet/issues/22
* https://github.com/Esri/esri-leaflet/issues/199
* https://github.com/Esri/esri-leaflet/issues/216

Everything else will have to wait until after Dev Summit so look another release later in March or April.