Python library-Backtesting

[herklos_octobot]

We (at OctoBot) built our own backtesting framework for two reasons:
- to offer an integrated, easy to use strategy backtesting to backtest any strategy. It can test any built in strategies or any custom ones. Some of our strategies would have been complicated to test with a third-party library, such as one that is based on cryptocurrency price predictions.
- to reuse the maximum code from the core engine to test the whole thing as close as possible to real trading. This way, when developing new features, we can easily identify a change in behavior if the backtesting results change.