Makefile for converting R Markdown files to Markdown

[jdblischak]

See #524 for discussion. See #598 for my previous attempt.

To observe the effect of these changes, run the following:

make rmarkdown
make clean
make site

And then preview any of the lessons, e.g. _site/novice/r/01-starting-with-data.html. The style should be consistent with the other novice lessons, e.g. _site/novice/python/01-numpy.html.

[jdblischak]

Thanks for all the help, @cboettig, @ramnathv , and @wking. Your suggestions greatly improved this pipeline. I think it is fairly straightforward after what we discussed. It uses knitr hooks to properly label code input and output blocks, and the Makefile knits the file in the directory where the file is located so that the figures are saved in the proper place. If you have time, could you please take a quick look to confirm before I merge this PR?

@gvwilson, I designed this to mirror the build process and the end result that we use for the IPython Notebooks. Is there anything else you can think of to improve the process?

[jdblischak]

Thanks to @gvwilson for his review via email.

[gvwilson]

Yay!