Add Makefile For Easy Automated Testing Immediately After Building

As extensively [discussed on the -docs mailing list](https://groups.google.com/forum/?hl=en#!topic/bitcoin-documentation/FMbBuFnJtJg), a 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.