Refactor previous commits

- Removing `AccountCurrency` from `Cash` and `Brokerage` classes.
`ICurrencyConverter` will now provide the `AccountCurrency`
- Adding new static `OrderFee.Zero` which will return a 0 order fee in
`NullCurrency`
- Adding static `Currencies.USD` value, replacing all "USD".
- Addin new static `Currencies.NullCurrency`
- Updating Bitfinex `FeeModel` so it return fees in quote currency.
Adding unit tests

Algorithm.Python/CustomModelsAlgorithm.py

@@ -103,7 +103,7 @@ class CustomFeeModel(FeeModel):
         self.algorithm.Log("CustomFeeModel: " + str(fee))
         return OrderFee(CashAmount(
             fee,
-            parameters.Security.QuoteCurrency.AccountCurrency))
+            parameters.AccountCurrency))
 
 class CustomSlippageModel:
     def __init__(self, algorithm):