REL: Release v0.6.0

Highlights

- Reworked risk metrics, including verification against Excel spreadsheet
- Additional order methods
- Conversion of many data structures to use pandas
- Change to behavior of stop and limit orders (@pcawthron)
- Use pandas timezone handling throughout instead of Delorean
- New commission model (@stanh)
- Adds beginning of support for Toronto stock exchange. (@dstephens)
- Python 3 compatibility.
  Unit tests now pass when run with Python 3.3, and Python 3 should
  now be considered officially supported.
  If anything does not work under Python 3, please file as a bug.