Feature add agol geocoder

This pull request adds support for the ArcGis Online Geocoding service provided by ESRI
( http://resources.arcgis.com/en/help/arcgis-rest-api/index.html#/Overview_of_the_World_Geocoding_Service/02r300000009000000/ )

Test coverage for .geocode() & .reverse() have been included

The version number for node-geocoder has been incremented to 1.4.0 to reflect this change.