Various bug fixes and cleanup.

* bug fixes:
  * fixed directory-existence argument check
  * fixed how `.urchin_stdout` files are cleaned up
  * fixed instances of error/warning messages outputting to stdout instead of stderr
* documentation, help and error-message improvements:
  * added specific error message if the (non-option) argument is not a directory
  * improved wording of CLI help
  * readme.md: replaced obsolete URL http://www.urchin.sh with https://github.com/scraperwiki/urchin
  * readme.md: made the fact clearer that `./cross-shell-tests` only tests urchin's _own_ cross-shell compatibility
  * HISTORY: fixed typo
* formatting and logging improvements:
  * added timestamp to the beginning of log files (`.urchin.log`)
  * captured output from failed tests is now printed in *red* to draw attention
  * test summary now prints the number of failed tests in the appropriate color for instant feedback (green, if none failed; red, otherwise)
* cleanup
  * removed unused test files, simplified some tests