ENH: define searchsorted on a Series

[jreback]

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)

[jreback]

@cpcloud ?