Fix portfolio management

[kylerberry]

• Portfolio stats are handled in a more sane fashion
• net and percentage gains/losses may actually be calculated correctly (finally)
• position is no longer opened or updated until its dependent order is matched
  • moved this logic to the 'match' event in the feed handlers
• add slippage option to the backtest results
• calculate avgLoss and avgWins
• fixed incorrect slow-stochastic calculation
• CandleProvider resets itself every config.granularity seconds (candle lows & highs were never reset causing calculations to flatten out overtime)