apache_request_headers compatibility

"Adding emulated version of apache_request_headers() function for compatibility with non-apache webservers. Ideally the dependency on this function would be removed, but that can be looked at later?"

Grabbed from here: http://uk3.php.net/manual/en/function.apache-request-headers.php#70810

Up to you whether or not you accept this commit - the alternative is to remove the dependency on apache_request_headers.

But with this bodge in place we're successfully running the framework on nginx.