Merge pull request ipython#1780 from jonathan-taylor/rmagic_extension

Rmagic extension to use R (the statistical package) seamlessly from IPython.

The rmagic extension allows R inline code as well as cell level magics. An example notebook is provided in docs/examples/notebooks/rmagic_extension.ipynb to demonstrate its usage.

Main points:

1) Allows capture of plots to R via inline png plots (like --pylab inline)

2) Allows capture of R's stdout() connection to the notebook

3) Allows simple push/pull for array data to/from R (via rpy2) with copy only on push to R -- this seems necessary.