Improved profit and loss reporting

[Hilko]

At first I had made a command line version of my PnL calculation. Soon I realized that I could not see my average open position on my mobile, on the go. So I had to run this my server, which requires a GUI.
For who is interested to have a look or develop it further, it is available in my pull request here. It is essentially a verbatim copy of my command line tool, with a GUI page on top of it.
This means the integration with the rest of the Gekko code can be improved.

•     * API keys need to be set using environment variables. It is not using Gekko's existing functionality for managing API keys.
  
•     * The new functions for fetching the trades from the exchange are in their own module. They could be merged into the existing exchange interfaces to keep this code organized in the place where it belongs.
  
•     * Since I don't have accounts on every exchange, it is not tested for other exchanges besides Kraken. Adding support for more exchanges should be fairly straight forward.
  

I think Gekko is really great, so I've decided to give this back, instead of keeping it to myself, as a way  to say thanks. I hope others find it useful too or make it better!