arithmetic truncation
expr: datetime precision strings
expr: datetime truncation operation
numpy: datetime truncation
pandas: datetime truncation
python: add datetime truncation functions

This might be appropriate as a separate library

This is not robust to timezones
python: datetime truncation in effect
mongo: floor and ceiling
spark: add missing isscalar import
spark: test numerical truncation
mongo: floordiv
py34: add epsilon to milliseconds computation
py34: 0\d -> \d
py26: timedelta.total_seconds compat function
numpy: datetimes truncate to monday
py26: skip rounding error on doctest
fix pandas truncate datetime test to match Monday
numpy and python datetime truncation point tests

cc @mwiebe
numpy: datetime truncation on datetime64 scalars
into: dtype_from_tuple works on homogenous types