Fixes #473, catches outdated player names and checks if all entries are UUIDs before converted and thus prevents the server crashing.

Split off logic into single responsibilities for storing, initializing and matching the UUID mappings. Added JUnit to create tests for the UUID Matcher to see if its legit. It tests against simple problems like unconform UUIDs and also tests against a valid UUID.
Merge pull request #480 from thatsIch/uuid

Fixes #473, catches outdated player names and checks if all entries are UUIDs before converted and thus prevents the server crashing.