Support for datetime columns with rpy2
Check to see if the values we're looking at are numpy datetime64
objects. If they are, we can assume we're looking at a Timestamp object,
which subclasses Python's datetime class, and we return a struct_time
which is understood by rpy2's POSIXct class.

Attempt at a fix for  #2351

Nice. I left those out when I wrote support for conversions mainly because I did not use any of the datetime features (on R and Python). As one who wrote the code, you get a +1 from me.
 Cherry-picked this commit. It'd be great to see some tests for this at some point (separate PR is fine)
