Added the code for the Base and Dummy exchange classes to the exchang…

[mwbrulhardt]

I have added the beginnings for the abstraction of the exchange and have incorporated them into the TradingEnv class. The only exchange that is implemented right now is the dummy exchange which is to be used for training and testing. Since I am missing the "data/agents/PPO2__MlpLnLstmPolicy__IncrementalProfit__1.pkl" pickle file at the moment I can't run it. I think it will run smoothly, however, definitely check that it does.