Update to Provisioning code and handling for HTTP_POST method

I added code to enable the draw_provisioning_button() to accept a challenge array (and thus provide for post-provisioning authentication), so you'll see changes in both OneID.php files as well as the provision.php file.

I also added handling that detects and re-formats if the HTTP_POST method is used with a callback URL.  This is pretty minor and likely not being used by anyone, so feel free to take it out.

Thank you,
Dan