Update filesystem interface to specify root storage directory in vtkMap.

vtkOsmLayer then uses subdirectory underneath that.
Note that default OSM cache directory renamed from "tiles" to "osm"
Add minimal test for vtkOsmLayer

Also updated vtkMap constructor to initialize default StorageDirectory
Check for curl error, and remove image file on error
Merge pull request #48 from OpenGeoscience/john/updated-directory-logic

Update filesystem interface to specify root storage directory in vtkMap.