Commented Task: Add Documentation to methods [2569]

Each public method should have a summary describing what the method is doing.
This should be the case for the TweetinviLogic.Models as well as Tweetinvi static classes.
Comments: Associated with changeset 45342: Version 0.9.10.0
- Added documentation to Account and Auth static classes
- Changed account to use FriendshipController instead of private field that could have resulted in multi-threading issues.