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.
```
git checkout ef16415525 ./ModStatistics.cs
git rm ./ModStatistics.sln
git commit -m "Reverting changes to project and solution"
```
Would create a commit which effectively backs out those changes, which you can then push and this pull request will be automatically updated.

(ef16415525 being the commit that's currently the head of `master` in Majiir's repo).