Add blocks to customize service provider requests.

With these blocks you can open requests in a UIWebView within app instead of safari.

Example:

```
AFOAuth1Client *oAuthClient = [[AFOAuth1Client alloc] initWithBaseURL:...];

MyLoginViewController *loginVC = [[MyLoginViewController alloc] init];
[m_rootViewController presentViewController:loginVC animated:NO completion:^(){}];

oAuthClient.onServiceProviderRequest = ^(NSURLRequest *request) {
    [loginVC.webView loadRequest:request];
};

oAuthClient.onServiceProviderRequestFinished = ^() {
    [loginVC dismissViewControllerAnimated:YES completion:nil];
};

[oAuthClient authorizeUsingOAuthWithRequestTokenPath:...];
```