New Examples and fixes

Hi, this commit contains:

* Add Esri ArcGIS Dynamic Map Layer example: `examples/esri-dynamiclayer-example.html`.
* Add Esri Legend Example  `examples/esri-dynamiclayer-example.html`.
* Add watch on url changes in Legend directive.
* Add Legend Helper.
* Add esri-leaflet bower dependency.

TODO:

* Watch on changes in array legend.