Fixes #21. Force runtimes to handle x-domain. Explicit flag to indicate support.

Only some web browsers can do cross-domain requests with "native" XHR. To
ensure cross-browser request always work we indicate the "native" XHR cannot
handle the request so it passes onto the runtimes (which may of course by
HTML5 which is still really native).

Some runtimes can fulfill the request (i.e. Flash) while others may or may
not be able to (like HTML5). So I have added a explicit capability to filter
out one that cannot handle cross domain.

I am unsure if Silverlight supports cross domain (my guess is yes). But I did
not add the flag until someone can verify.
XHR: Fallback to runtime for cross-origin requests.
XHR: Small cleanup.
Runtime: Add cross-origin request making capability: do_cors. As suggested in #22.