Arcgis Fixes

Hi lads,
I have been working with clients trying to resolve some issues they were having with loading some of their ArcGis layers in osgearth. Main feature requests/additions were being able to specify which layers to load instead of the default of all of them & being able to load a layer with a wkt string for a spatial reference instead of only an integer wkid. I also extended the extent check as not all layers contain a fullExtent attribute in the json returned which was causing the loading of the layer to fail (0 for min.max/x/y values). Summary below:
 
- Add support for loading individual layers (comma separated string) via a <layers> option for
Arcgis image layers. Default is still _allLayers if none specified in the earth file.
- Add support for loading a layer if it uses a wkt string for the spatial reference.
- Fix an issue where if a layer did not contain a fullExtent attribute, then it would fall back looking for an extent or initialExtent to determine the min/max/x/y values.

Check out the changes & have a play with them. I've incorporated them into our local copy of the project but would prefer to have them integrated into osgearth as i think they will extend the capabilities of the ArcGis plugin nicely for other users.