Add known exchange balance addresses to address API

[bedeho]

Background

#38

Proposal

Add a new dictionary called known_exchange_balances which holds some representation of each known CEX, its set of known balances and total balance. It may be woth adding a sort of "disclaimer" return value here, so that anyone looking at API visually can learn that this is a best effort API value, and that we cannot be sure we know all exchange balances at all times, or even all places $JOY trades.