Adding json-specific matchers

These make the JSON tests much easier to read and
write. They're the first custom matchers that I can
find, so they're breaking a bit of new ground,
but the JSON tests were pretty hard to read
and there was a lot of duplication, so it seemed
worth it.

Note that for some reason they're not working on
Facts - it seems to get immediately turned into
a full instance by the JSON parsing subsystem,
and I've no idea why.

Reviewed-by: Daniel Pittman <daniel@puppetlabs.com>
Signed-off-by: Luke Kanies <luke@puppetlabs.com>