Patch 1.1: New impossible achievement, many audio bug fixes

Behold Battle Patch 1.1: - Fixed various unit attack and death sounds being buggy when too many units were attacking or dying at once. - Added steam achievement to beat the game on impossible. - Necro rat can now appear both in small battle size and medium battle size (was only in small before). - Game mode selection screen: changed from green highlight to arrows to make it clearer what is selected. - Medium battle size now unlocked from the start.