Arcgis Fixes

- Add support for loading individual layers via a <layers> option in
arcgis image layers
- Fix issue for failure to load a layer if it uses a wkt 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.
Merge pull request #479 from ib862/master

Arcgis Fixes