es6 refactor and other updates for 2.0.0-beta.1 release

[jgravois]

similar to Esri/esri-leaflet-heatmap-feature-layer#2

• depending on new Leaflet.markercluster tag
• rewritten in ES6
• L.esri.Cluster.clusteredFeatureLayer() (breaking change)
• moved to semistandard/ linted
• ditched grunt
• made a few small tweaks to get things working with Leaflet 1.0.0 b2
• bumped version numbers
• updated tests
• manually tested a few marker and cluster events to make sure they were firing

[patrickarlt]

@jgravois can you update the changelog? Then this is good to go and we can do a release.

[jgravois]

changelog created. and updated.