(#10) resolve widget.json url relative to spec dir

Reason: We can't expect the test to run at a specific
url/path, so expecting the `widget.json` to be present at the
parent directory of `window.location.path` will most likely break.
Merge branch 'feature/10-fix-widget-json-loading-in-karma'

* feature/10-fix-widget-json-loading-in-karma:
  (#10) resolve widget.json url relative to spec dir