Update Backtest function to work like Live Trading

[davidlatte]

Current BackTest code has its own special function that creates objects in a weird manner that the user has very little control or influence over. Instead, add a new flag 'backtest=True' to the Trader.run_all() method as the primary way to run backtest simulations. This will make the interface much more consistent for the user ... and it is what is already happening under-the-hood anyway.