Make mine erasure/saving less brute.

Before, I'd delete any .mine.yml files that I currently don't have in memory. However, if a mine was painstakingly setup by the user in memory, saved corrupt, and server is shut down, the mine gets erased because it wasn't loaded into memory.
Softdepend on Multiverse to make sure worlds are loaded.

The issue Multiverse users were having was that Bukkit only loaded overworld, nether, and end before it loaded MRL. Since Multiverse booted after MRL, I didn't have access to any Multiverse worlds.