Updated test suite.

Refactored the database connection to allow for a test database. Configured FactoryGirl and added two factories. Updated guard to set the "DB_ENV" var before running spec.
Team slugging command.

Setup associations between Moneyball::Player and Moneyball::BattingStat. Added methods necessary to compute a player's and team's slugging_percentage.
Added method to calculate batting average.
Refactored team_slugging_percentage command to return a more descriptive string.