Match nested json without a specific path

This has some things in common with #64 but It's a little bit more flexible and less rigid an option, for when you don't want to over-specify the path to the json, but simply want to test it's presence somewhere in the structure.  It's implemented as a subclass of IncludeJson, so I wanted it to pass almost all the same tests plus a few new ones, but as a result the tests are not fully DRY since it's a new matcher, and DRYing would reduce readability a lot.  If there's a better solution, including removing some of the tests from the new spec file, I'm happy to change.