Added -s option for running tests that have shebang line #!/bin/sh with the same shell that is running urchin itself, to support cross-shell compatibility tests.

Furthermore:

* CLI help, read-me updated to describe new option.
* Output captured from failing tests is now printed in red.
* Summary line stating the number of failed tests is now printed in red, if at least 1 test failed.
* XSD-related tests deactivated, since corresponding functionality is not implemented.
* other small fixes