Improving GDAL/OGR handling in extractorapp

- Check if native libs are available at bean instantiation
- Excluding some redundant jars at build time: have twice the gdal/ogr
  bindings could lead to version issues. In addition, we should use the
  generated bindings coming with the targeted native libs.
- Better logging if unavailable libs.

TODO:
- Propagate the modifications to other concerned webapps (mapfishapp,
  geoserver).