Adding four new instructors to the roster
Adding Steven Koenig (and fixing an HTML error along the way)
Merge pull request #31 from gvwilson/adding-more-instructors

Adding more instructors
Putting background image where it belongs
Adding explicit rules to copy image files to Makefile.  This is needed
because some versions of Jekyll only copy image files that are
referenced by HTML pages, and *not* image files that are referenced
by CSS (like our background image).  Safest thing is to always check
everything, even though it slows the build down by about 10 seconds.
Taking out .htaccess rewrite rule that was turning /blog/yyyy/mm/*.jpg into /blog/yyyy/mm/*.jpg.html
Copying only one image file manually to shorten build times
1. Modified Makefile so that if `v3` and `v4` submodules aren't present, site will still build.
2. Improved documentation for `bin/categories.py` to explain proper usage.
3. Added complementary `bin/authors.py` and Makefile target to display list of blog post authors' names.
Fixing formatting of names in person includes