move datetime stuff out of _arith_method
cleanup around DateTimeIndex
no need for datetime index right return
simplify checks for type conversions/views in time arithmetic
more timedelta refactoring
restore datetime : datetime special case
add np1.6 check for integer --> timedelta conversion under the hood
squash together cases just a bit more
add explanatory comment
short circuit index checks if same index
move all time operations for Series into class _TimeOp