Update Makefile, add example, use NAN, fix bug

--Makefile now includes a test target which runs each example program
  through valgrind
--The new example program tries to print out the result of a boolean
  comparison between an undefined variable and 3.
--NAN is much better to return from functions with a double return
  type than INFINITY for representing the NULL-so-abort value. Only
  problem is the call to isnan.
--Through the test I added I found a bug where if evaluateValue
  returns NULL to evaluateList, evaluateList keeps going along happily