REL: v0.5.7

Highlights, with thanks to contributors inline:

- Runtime performance improvements
- Fixed the omission of peformance messages on days with no trades
- Changes to batch_transform implementation
-- supports sid filtering
-- performance improvements using pandas
-- added an option for only computating when there is a window length's
   worth of data
- Added new risk metrics
-- Sortino
-- information ration
  (Ryan Day, ryanday2@gmail.com @rday)
- Added stop and limit orders
  (Tony Worm, verdverm@gmail.com @verdverm)
- Added variable recording
- Deprecated market_aware and delta kwargs to EventWindow
- Fixes to trading calendars for missing holidays
- Added TradingEnviorment context manager
- Added support for streaming through dividends
- Yahoo source now has OHLC
- Updates downloaded benchmark and treasury data when new data is available.
  (Ryan Day, ryanday2@gmail.com @rday)
- Added optional adjustment of Yahoo data
  (Jeremiah Lowin, jlowin@lowindata.com @jlowin)
MAINT: Moves internal state variables in performance tracker.

Slight refactoring of grouping the tracking variables in the
PerformanceTracker together.

So that it's easier to see which are config members and which are
members used to track internal state.