Manual OAuth Authorization Step

This PR is based on #5.

Allow the XNGAPIClient consumer to handle the authorisation step manually and show a web view instead of opening Safari automatically.
This can prevent rejections like this one:
>10.6: Apple and our customers place a high value on simple, refined, creative, well thought through interfaces. They take more work but are worth it. Apple sets a high bar. If your user interface is complex or less than very good it may be rejected
>We found the following issues with the user interface of your app:
>After the app is launched, it opens a web page in mobile Safari first, then returns to the app. The user should be able to use the app immediately upon launch.
>These examples identify types of issues discovered in your app but may not represent all such issues. It would be appropriate to thoroughly evaluate your app to address these types of issues.