Add support for empty geocoding Task constructors.

as mentioned in my #49 comment.  it seems helpful to just assume people want to use the World Geocoding Service so that developers can just instantiate a Task object with an empty constructor.

ie:
`L.esri.Geocoding.Tasks.geocode()`

made code change and added a relevant test.  
also updated a reused variable in GeoSearch.js to make jshint happy.

let me know if this is unwanted or if there is a more desirable approach.  if its all good, i can update the doc as well.
@jgravois why is this needed again? Rather then having a special case for instantiating these from controls we should probably just fix how the control creates these.