Putting template into final [sic] form

1. Moving Markdown files into `pages` sub-directory.
2. Creating Makefile to compile upward into root directory.  (Need a Makefile because we're using Pandoc to build pages ourselves rather than relying on Jekyll.)
3. Separating `fig` (figures used in lesson) from `img` (web site artwork).
4. Creating `_includes` and `_layouts` (using same names as Jekyll even though we're building with Pandoc).
5. Cleaning up explanation of lesson template.