Opt-out option in dialog

Adds a new radio button to the first-time popup that also asks the user if he wants to enable the statistics collection. I also moved a couple of checks in the start method so that the popup is executed first.

If I understand how the unity GUI stuff works, it *should* start enabled (it does in my install).

You will also want to edit the text in the popup I guess.
I still prefer to have the deps in source control (or better yet [NuGet](https://www.nuget.org/)), but alas. :smiley: