ENH: Add calendar and test for Toronto stock exchange.

Note that the calendar test is decorated with @nottest (as per the other calendar test functions).  I've run the test to confirm the calendar works.  The differences between the env (Yahoo Finance of GSPTSE) and the calendar are illustrated in the tradingcalendar_tse file and are confirmed to be errors on Yahoo Finance's part.
ENH: Added functionality to download Canadian treasury curves.

Added automatic switching of treasury curves based on index sent to environment.
MAINT: Update URL for free benchmark data.

Keep pace with Yahoo!'s change from ichart.yahoo.com to
ichart.finance.yahoo.com