Removes unused parameters in test trade info creation.

This code hasn't used `random` in a while, removing the parameter
left as an artifact.
Adds dictionary style access to Position object.
Adds dictionary style access to Portfolio object.
Fixes dictionary-style lookup on Position and Portfolio.

Typo on method name, /__getattr__/__getitem__
Add the Sortino ratio for downside risk
Force float value, and compare result against boundary
Merge pull request #66 from rday/sortino_ratio

Add the Sortino ratio for downside risk
Removes unused method.
Merge branch 'master' of github.com:quantopian/zipline into record_vars