Add to blankUrl option as a fallback for executeCrossDomainPost

It enables to safely fallback from `executeCrossDomainPost` to `crossDomainPost` with a non-default `blankUrl` setting.
Testing `executeCrossDomainPost` with and without `blankUrl` option.
Merge pull request #163 from oncletom/patch-2

Add to blankUrl option as a fallback for executeCrossDomainPost