Push pandas dataframes to R magic

At present, if you make a pandas DataFrame and try to use it with rmagic, the converter spits out some ugly nested lists without column names. The obvious expectation is that it will produce an R data.frame, which the design of DataFrame was based on.

The only concern with this is that the [pandas docs](http://pandas.pydata.org/pandas-docs/stable/r_interface.html#converting-dataframes-into-r-objects) describe the conversion as 'experimental', and from a comment in the source, it won't handle multi-level indices. We could catch errors and fall back to the current behaviour, but the result seems fairly useless, and I think it's more confusing to silently change behaviour when there's a problem.