Fix trading quirks

Multiply with BigNumber to prevent errors.
Clear the order book for invalid currency pairs (same fix I added to /trade to clear the orders table when the pair is invalid).
Delete the currency's issuer if it is undefined/falsy (```_.isEqual({currency: 'STR'}, {currency: 'STR', issuer: undefined}) // => false```)
Deep compare currencies to determine if they have changed.