(#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-0.9.x/10-fix-widget-json-loading-in-karma' into release-0.9.x

* feature-0.9.x/10-fix-widget-json-loading-in-karma:
  (#10) resolve widget.json url relative to spec dir