Added 4 new difficulty modes to unlock, now up to Difficulty +5.

Buffed some perks, nerfed some perks.
Also added a new Fire perk that allows fire damage to stack. Let me know if it's OP or shit.

Enemies now spawn much less quickly after the boss appears.

The issue of the player "teleporting" during lag spikes (usually into a group of enemies) has hopefully been fixed. Thanks to eEight for pointing out that low fps affected movement speed, because player deceleration wasn't correctly timeScale independent.