Promote closed_positions() to standardized method

[deepbrook]

Add closed_positions() as a standardized method. It should return a list of executed trades from the exchanges private API. It likely has a different name at each exchange.

• Assert that closed_orders() is available on each supported exchange (Trade history, closed positions or similar)
• Create PairFormatter Unittest
• Create Endpoint Unittests
• Implement Endpoint in RESTInterface class
• Implement Endpoint in Exchange Interfaces