Add ability to pass options object into layer#identify

This pull request adds the ability to pass in an options object to layer#identify. The particular use case I had was restricting the identify by layerDefs against ArcGIS Server 10.x. There is a demo added to showcase this functionality. For the sake of simplicity, this does not accept layerDefs as an array (only object or string), though this would be easy to implement if needed.