PATCH: Add hstore_to_json()

I just realized that this was easy to do, and despite my complete lack of C skillz was able to throw this together in a couple of hours. It might be handy to some, though the possible downsides are:

* No json_to_hstore().
* Leads to requests for hstore_to_yaml(), hstore_to_xml(), etc.
* Andrew Gierth said “no” when I suggested it.

But it's kind of handy, too. Thoughts?

Best,

David