Add version number to save file produced

[JLLeitschuh]

The version number should be saved in the grip file that we produce.
This will keep us from trying to import a save that we don't support anymore.

[SamCarlberg]

Shouldn't we be able to upgrade old saves? Or at least try, and alert the user if it's incompatible.

[JLLeitschuh]

Yes. I agree. We just need to be able to tell what version of grip generated the save.