Allow use of ApiRequestor for Stripe Connect

Hi guys,

Firstly, I'm loving Stripe; nice work!

Secondly, I'd like to be able to make use of your ```ApiRequestor``` class to make the post to ```https://connect.stripe.com/oauth/token``` while trying to connect users.

I notice that the ```ApiRequestor::request()``` method takes a ```$url``` argument. Unfortunately, this can only be a URL relative to ```https://api.stripe.com``` and so we can't use it to post to ```https://connect.stripe.com/oauth/token```.

I propose updating the ```ApiRequestor::_requestRaw()``` method to check whether the URL is already an absolute URL. If it is, we use it as-is. Otherwise we append it to the base API URL (existing behavior).  

    // ...
    // Check if $url is an absolute URL (over SSL)
    if (preg_match("#^https://#", $url)) {
        // It is, so use it as-is
        $absUrl = $url;
    } else {
        // It's not, so append it to Stripe::$apiBase
        $absUrl = $this->apiUrl($url);
    }
    // ...

We can then use it as follows:

    $url = 'https://connect.stripe.com/oauth/token';
    $request = array(
        'client_secret' => MY_SECRET_KEY,
        'grant_type'    => 'authorization_code',
        'client_id'     => MY_CLIENT_ID,
        'code'          => $_GET['code'],
    );

    $requestor = new \Stripe_ApiRequestor(MY_SECRET_KEY);
    $response  = $requestor->request('post', $url, $request);

Let me know what you think.

Cheers,

Tom