Don't spawn bosses after they've been defeated
Fix bombs by making bomb tick a passive effect

This prevents bombs from needing to "target" enemies in order to use
their tick power.

For enemy bombs, we need to mark them with "combat_style=aggressive",
because enemies that leave combat are automatically healed.
Increase the chances that Zalkaz will spawn small bats
Merge pull request #76 from dorkster/fixes

Fix bosses and bombs