Fix the management of external SLD styles 

This PR introduces an important change that authorizes back the use of external SLD files in the wms server, as described in #14.  Unit tests have been added to check that:
- The parameters are tested the right way (by myself)
- The processing return a valid wms response (by Erwan). This test has proven to be really efficient, even if it is more an integration test than a unit test. Thanks @ebocher ;-)

@ebocher proposed some code to improve Exceptions management. I kept it as I think it is a good idea.