Add feature policies to control execution in iframes

Provide a mechanism to declaratively control how iframes are frozen under
two situations:

1. when iframes are not rendered;
2. when iframes are scrolled out of the view.

This commit also removes the idea of frozen task sources, as it turns out all tasks should be frozen for frozen documents; we were confused by the fact that some implementations use similar internal machinery for spec-tasks as for non-spec tasks. Closes #20. Closes #27.

Author: dtapuska