Changes necessary for Xamarin.Auth to connect to DropBox with OAuth1

The OAuth1Authenticator needed a small tweek in order to get it to
connect to DropBox. I'll let you decide if you feel like this tweek
is acceptable; however I thought I'd share it.

Additionally, I added to the iOS sample to demonstrate how to connect
to DropBox with OAuth1.

Contribution from Jon Lipsky: https://github.com/xamarin/Xamarin.Auth/pull/3

Reformatted to keep the whitespace in place, and properly format.