Reduce build warnings

@timheuer 
This pull request reduces the number of warnings to 108 (down from 275).
There are no code changes. Two things were changed:
- Added pragma ignores for when obsolete Callisto types were used.
- Added xml doc comments to most classes.

What's left: 
 - XML doc comments to mainly OAuthHelper as well as a few other classes. 
 - Resolve other obsolete warnings due to changes in WinRT8.1.