Keep track of balance

[jonas-lj]

The Capybot should keep track of the balance of each token it's trading and also how much it have spent on gas and fees. This will help traders to evaluate how the bot is doing. Assuming we can get pricing data for all tokens, it could also compute a total balance as a single value, say USD.

These numbers should be logged so the Capybot Monitor can produce plots showing the development.