Added missing fields to GitHubCommit

Added `Stats` and `Patch` fields to `GitHubCommit` and `GitHubCommitFile`, repectively. Also, modified the type for the `Signature.Date` field since it was `DateTime` and everywhere else in the project uses `DateTimeOffset`. 

PS. My stupid Mac or stupid Xamarin instance won't build the test projects for some reason I have yet to debug so don't be surprised if I fail the continous integration build :sunglasses: