Move Response building logic into separate function

[griffy]

This was done mainly to allow for an Httpful\Request to be used with curl_multi_exec like so:

$mh = curl_multi_init();
$requests = array(...);
foreach ($requests as $index => $request) {
    curl_multi_add_handle($mh, $request->_ch);
}

do {
    $status = curl_multi_exec($mh, $running);
} while ($status === CURLM_CALL_MULTI_PERFORM || $running);

$responses = array();
foreach ($requests as $index => $request) {
    $responses[$index] = $request->buildResponse(curl_multi_getcontent($request->_ch));
    curl_multi_remove_handle($mh, $request->_ch);
    curl_close($request->_ch);
}

Not sure if true curl_multi_exec support is desired or within the scope of Httpful, so this is a quick workaround for now.