DynamicMapLayer and identify improvements

This implements some of the improvements suggested in https://github.com/Esri/esri-leaflet/issues/138, including...

1. `identify` will only use the layers you specify in the layers option
2. `identify` will use the size of the map (instead of 800,600,96)
3. `tolerance` is now `5` by default. Was `3`.

This also fixes https://github.com/Esri/esri-leaflet/issues/141 so `DynamicMapLayer` should work with `L.Control.Layers`.

Thanks @Atlmapper @traderboy for the suggestions and bug reports.