Added Link Accounts Method

I added a method to the Auth0Client class linkAccountAsync:connection:scope:auth0AccessToken:withCompletionHandler: for linking other accounts to an already existing auth0 Account. 

I also changed the return URL check to be case insensitive. More details on this:

I had been having issues testing Twitter login when I set my namespace to something like this: 'NameSpaceName.auth0.com'. No errors, but the callback wasn't being called. After debugging, I found that in webView:shouldStartLoadWithRequest:navigationType: inside the Auth0WebViewController, the requestURLString was coming back all lowercase. 

Now that I'm starting to understand Auth0, I see that it's the return URL I set in the dashboard application settings. But in order to fool proof this for future users, I set the return URL check to case insensitive, in case there are capitals in the namespace from instantiating the auth client.