fix image loading over https

[troosan]

this fixes issue #983
As images are now loaded using file_get_contents, it is possible to modify the stream context using stream_context_set_default(...).
In ImageTest I'm using

        $arrContextOptions = array(
            "ssl" => array(
                "verify_peer" => false,
                "verify_peer_name" => false,
            ),
        );
        stream_context_set_default($arrContextOptions);

which you should obviously not do in a production environment, instead you should pass the certificate to use to connect to the remote url:

        $arrContextOptions = array(
            "ssl" => array(
                "cafile" => "/path/to/bundle/cacert.pem",
                "verify_peer" => true,
                "verify_peer_name" => true,
            ),
        );
        stream_context_set_default($arrContextOptions);

set thread on stackoverflow