jansson: add embedded version of Jansson 2.6

Currently, we only support building the internal Jansson.  If desired,
we can add the ability to compile against an external Jansson someday.

To integrate a new version of Jansson someday:

1. Copy in a new set of files into the jansson directory.  Do not
   include any generated files!
2. Copy from jansson/configure.ac to jansson/jansson.m4 -- read the
   comments in the current jansson/jansson.m4 to see exactly how to do
   this (jansson/jansson.m4 is basically configure.ac with a bunch of
   stuff stripped out).