ENH: define searchsorted on a Series
http://stackoverflow.com/questions/22669208/attributeerror-series-object-has-no-attribute-searchsorted-pandas

essentially a pass thru to `np.searchsorted`
- should return the index (rather than indices)
- can better handle `datetime64[ns]` (e.g. coerce input to/fro)
- could check if sorted internally (e.g. `is_monotonic` on the index), other pass the `sorter` argument)
