Remove unused import shutil
Fix the next level existence checks
Import all units to level loader

Since we dynamically load any units that are part of the level, we need
to import all possible units for the level loader to use
Add tower level 2
Fix abilities given in towers 1 and 2
Fix the base unit's 'say' method
Add play_turn for sludge
Fix take_damage on unit_base
Fix unit detection in ability base
Fix damage application typo in ability base
Add a fallback earn_points method to unit base

So that when sludge kills the warrior, it doesn't blow up on undefined
method
Fix unit retrieval in floor
Fix template README
Add descriptions to abilities
Add tower 3
Add Rest ability
Fix the displaying of level clue