Add the basics of the Account Manager.
Plumb the account manager through to all the features

They should be able to access player accounts.
Join and quit events should run through the Account Manager

The Account Manager will decide when it is ok to tell features about a
certain player, which we should only do when their account is available.
Load and unload accounts when Mineground gets loaded and unloaded

Act as if all players who are online when the Mineground plugin gets
loaded or unloaded just joined, and inform the account manager and all
the features as if that were the case.