Several small bug fixes.
Several small bug fixes.

- url_getContext():
  forgot to skip "://" in the URL string;
  the offset of the path context was calculated in a wrong way

- url_new4():
  misuse of 'sprintf' caused Segmentation Fault

- url_getContent()
- url_isaURL()
  give these two function a quick but fake implementation, because they
  will otherwise always return a GDK_FAIL without any further explaination,
  which is quite comfusing. Real implmentation will need much more time.

zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb