Updating files to move  into root directory

After discussion, getting rid of the `pages/` directory and putting everything in the root so that file paths to `fig/`, `data/`, etc. will be the same in source and generated pages.  This doesn't matter much for simple Markdown, but is important for R Markdown, IPython Notebooks, and other "live" pages.

> So notebooks are allowed as primary content again? What's to stop all 
> the lesson content from ending up in a notebook?
>
Nope, the primary content must be Markdown formatted the way we ask - if 
people want to also have Notebooks, .rmd, or the like, that's completely 
up to them.  Our tools won't validate them, our standard Makefile won't 
build them, etc.