Removing System.loadLibrary() calls

Remove calls to System.loadLibrary (GDAL only, I was not sure on how to rework other ones)
    
This PR follows the one made on GeoTools here:
https://github.com/boundlessgeo/geotools/pull/15
    
Calls to System.loadLibrary() should be done by the GDAL/OGR java bindings (gdal.jar, which most of the time is provided by the distro). With some specific classloaders (e.g. tomcat), a second call to loadLibrary using the same lib as argument raises an exception (UnsatisfiedLinkError: "already loaded in another classloader"), so even if we can make use of the native GDAL lib, it is reported as being unavailable.

Of course, comments are welcome