API: standard deviation bars optional in create_event_returns_tear_sheet
API: standard deviation bars optional in create_event_returns_tear_sheet

Sometimes the user just want the event study style return plot without
the additional sub-plots that show the standard deviation bars.
Added an option to allow the sub-plots to be optional.

quantopian/alphalens,luca-s/alphalens,quantopian/alphalens,luca-s/alphalens