REL: v0.5.9

Highlights:

- Benchmark updating now permits empty ranges.
  (Fixes runtime crash when running immediately after Easter 2013.)
- Risk metrics
  - Performance improvents from converting to numpy and pandas.
    <@wesm, wesmckinn@gmail.com>
  - Refactoring of risk metric calculation out of class structure.
MAINT: Rename risk metric function names to include '_ratio' suffix.

To make the risk metrics being calculated more clear, change the
naming convention that ratios have a '_risk' suffix.

Also, fixes typo in beta docstring.