Added -s option to support cross-shell compatibility tests.

Specifying `-s` now causes urchin to invoke tests that have shebang line `#!/bin/sh`  with the same shell that is running urchin _itself_. See updated `readme.md`.

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