Add Nightwatch command to fetch a Drupal URL directly with headers

Added new helper commands for Nightwatch tests:
- thDrupalFetchURL() - to fetch a Drupal relative URL directly, with headers and cookies, but in the background, without touching the browser state.
- thCacheClear() - to clear Drupal cache - full, or per tag, per bin.
- thLogout() - logs out the current user.

Automatically closed - issue fixed for 2 weeks with no activity.