Adds a trading day index to tradingcalander.
Adds a trading day index to tradingcalander.

The trading day index is all business days in range minus the
non trading days we are already calculating.

Also, uses trading calendar indexes for batch transform, since the
batch transform was the only use of non_trading_days.

Instead of constantly adding and removing holidays to do market
day delta math, uses pandas DatetimeIndex to get the index of the dates
and uses the index difference to calculate market days.

quantopian/tradingcalendar