convert_to_r_dataframe missing conversion for datetime values
I am in desperate need of converting my datetime values into something R can use so I that I can plot some data with ggplot. It appears that rpy2 supports this conversion but I am not sure how to go from a column with a datetime field (stored as a `numpy.datetime64`) into a `time.struct_time` value.

The relevant information about rpy2 is here: http://rpy.sourceforge.net/rpy2/doc-2.2/html/vector.html?highlight=intvector#rpy2.robjects.vectors.POSIXlt

I see that there need to be some minor changes [on line 244 of `common.py`](https://github.com/pydata/pandas/blob/master/pandas/rpy/common.py#L244) but basically I am stuck on trying to figure out how to convert from `numpy.datetime64` into the `struct_time`. I see that there is already functionality in Pandas, specifically the `to_pydatetime` function, for converting from this numpy format into a `datetime` object.

If someone could lead me over this hump I'd be happy to submit the pull request.
