Pre RC 1 updates

#### Events for layers
https://github.com/Esri/esri-leaflet/issues/131

`L.esri.FeatureLayer` and `L.esri.ClusteredFeatureLayer` now have the following events.
* `loading` - fired when features start loading from the service. Will pass `bounds` with the event 
* `load` - fired when all features are visible for the current map bounds. Will pass `bounds` and `cells` with the event
* `metadata` - fired when metadata for the service is available. Will pass `metadata` with the event
* `render` - fired when a feature is shown on the map, either from a new load or re shown on pan/zoom. Will pass `feature` and `geojson` with the event.

`L.esri.DynamicMapLayer` and `L.esri.TiledMapLayer` also have the `metadata` event.

#### New Tile Sets
There are several new options for `L.esri.BasemapLayer`.

* `ShadedRelief` - A worldwide shaded relief map.
* `ImageryTransportation` - A layer of street networks for overlaying on the `Imagery` map
* `ImageryAlternateLabels` - An alternate layer of labels for pairing with the `ShadedRelief` map