added current_week method, and constant for season start date
moved the start date up a couple of weeks

Even though the 2013 season started on September 5th, the rankings data starts coming in on the Tuesday before the game.  This change reflects that.
if not in season, instead of puts-ing a message, an error is raised
added Timecop gem for mocking Time requests
added some #current_week specs using Timecop
Merge pull request #3 from kelmerp/current_week

Current week