Fix multiple players being able to log in. Fixes #150
Kick current player instead of preventing login
Moved logic to the correct place
Revamp login logic. Fixes #150
Merge with GlowStone upstream
Cleanup and whitespace
Merge branch 'master' into login
Merge branch 'login'

This merges changes from pull request https://github.com/GlowstoneMC/Glowstone/pull/153.