ENH: Add support for converting DataFrames to R data.frames and matrices
Finally I got around to implement this properly in pandas, and I added also a conversion to R matrices if need be. Notice that this doesn't handle MultiIndex and similar functionalities, but since I don't use them I'm not sure how they'd fit into R data frames.

The biggest drawback here is the use of an intermediate, but other solutions would probably involve hacking around the rpy2 internals (so far all the alternative solutions I found where worse, performance-wise).

Two (smallish) unit tests are included. This code has also been extensively used in production where I work. 

All other unit tests pass after this change.

closes #350
