Fixed crash when killing Minecraft if it's already closed.
Fixed MMCFOUR-5: https://jira.forkk.net/browse/MMCFOUR-5
Switch from MCNostalgia to MCRewind.

This is mostly just a quick, hacky fix to switch over to the new system.
Merge git://github.com/Kyso/MultiMC4
Merge https://github.com/Lunatrius/MultiMC4
Merge branch 'master' of https://github.com/peterix/MultiMC4