get_past_trades requires an optional argument

[natehawkboss]

Problem: It looks like the get_past_trades method requires the symbol as an argument, but the gemini API does not require this. Doing it this way requires a user to make several requests if they are checking the status of orders across multiple currencies, or to completely sidestep the get_past_trades and directly call the api_query method (which I did).

Solution: make the symbol an optional keyword argument.