Set unsafe-content-script to true in package.json

I patched the addon according to https://blog.mozilla.org/addons/2014/04/10/changes-to-unsafewindow-for-the-add-on-sdk/ as you advised in https://github.com/klemens/ff-youtube-all-html5/issues/26 for an interim solution for this issue.

Feel free to ignore it if the real fix is ready soon.