R data frame

This PR implements a custom python => R converter. The behavior is exactly as before (call to np.asarray), except in case that the python input is a pandas DataFrame object. In that case, we preprocess the dtype of the resulting struct-array, to account for the fact that pandas casts strings into 'object' dtype. Here we will assume that if you got something with 'object' as it's dtype, you want it to be a string. 

See example here: 

http://nbviewer.ipython.org/urls/raw.github.com/arokem/ipython/r_data_frame/docs/examples/notebooks/rmagic_extension.ipynb

This still seems somewhat limited in scope (what if you want the 'object'), so I would be happy to get feedback on this.