Revert "Changed difficulty system so enemy spawns are no longer dependent of it."

This reverts commit 5e097514f5e3781d6ba771bba7de56d9dabf5f79.
Fixed some variable names
Merge branch 'master' of https://github.com/rikproDVC/Gradius

Conflicts:
	Gradius-csharp.sln
	Gradius.sln
	Gradius.userprefs