Hey! Integration Tests!

Our integration tests have been kinda lagging behind everything else. So I sat down and wrote a bunch today - particularly around the Git Database API.

This highlighted a couple of pain points with the API, so I fixed them.

Of note:

 - [x] 15 more tests 
 - [x] folder structure now mirrors `Octokit.UnitTests`
 - [x] removed some namespaces around test classes
 - [x] we missed some properties on `IGitDatabaseClient` were present
 - [x] `NewTree.Tree` needed to be populated. I'm lazy so I did that in the ctor.
 - [x] when creating a tree you need to specify the `Mode` property otherwise it totally freaks out when using it in a commit. 
    - I now validate this inside the request and throw an exception. 
    - Our API Ninjas are looking to deploy a fix for the Trees request succeeding when it should in fact fail...