Add Makefile For Easy Automated Testing Immediately After Building

New Makefile lets testers build the static HTML files and then run a few
automated tests to catch problems like missing files and links.  The
Makefile is not required; the site can still be built using just Jekyll.

This commit also fixes several image-related errors reported by `make
test` by adding the missing PNG images.
Merge remote-tracking branch 'harding/makefile' into merge