Redo a bunch of the retriever
Redo a bunch of the retriever

- Add user agent (some sites behave weird if they think you're not a
user)
- crossorigin.xml -> crossdomain.xml
- Disable TLS check
- Bug fixes for sites that return 404 and text/html for .xml files
- Add CORS preflight check (with Origin)

mozilla/http-observatory,april/http-observatory,april/http-observatory,mozilla/http-observatory,mozilla/http-observatory,april/http-observatory