(bug 19907) Adds support for cross-domain AJAX requests to the API.

(bug 19907) Adds support for cross-domain AJAX requests to the API.
Uses the Access-Control-Allow-Origin header for browsers that support it.
<http://dev.w3.org/2006/waf/access-control/>
$wgCrossSiteAJAXdomains can be set to '*' to allow requests from any domain,
an array of domains to allow, or, if $wgCrossSiteAJAXdomainsRegex is true,
an array of regexes to match against the request origin