R magic using knitr

This is a rewrite of the %R magic to produce output that is more like with [knitr](http://yihui.name/knitr/) output.

* R's help can now be accessed in any cell / line that begins with "help" or "?" with an attempt to produce
stderr when R cannot find help.

* Instead of python exceptions, R's stderr is published.

* The R magic no longer returns anything, but objects can be accessed through -o, -d options, and passed to R through the -i options.

* Wishlist item: if ipython had a markdown mime valid for publish_display_data, then some cells could be passed and returned as markdown through knitr.

* Also, see https://github.com/jonathan-taylor/ipython/pull/new/Ronly for a dedicated R notebook which just passes every cell through the R magic -- only 1 line of code is changed...

* It could be run through a pipe rather than rpy2...