pg: Add json, jsonb, jsonpath support

NOW we're really getting to the part where this module is more awesome
than DBD::Pg.

(When I started working on this module I was expecting that the Postgres
binary protocol would send jsonb in a binary format as well and that I'd
be duplicating parts of the JSON parser/formatter to make that work, but
it turns out that Postgres just uses plain json for exchange. Saves me
some trouble, I guess)

c/pgconn.c

@@ -406,7 +406,7 @@ static void fupg_results_setup(pTHX_ fupg_st *st) {
     st->recv = safecalloc(st->nfields, sizeof(*st->recv));
     if (st->stflags & FUPG_TEXT_RESULTS) {
         for (i=0; i<st->nfields; i++)
-            st->recv[i].fn = fupg_recv_textfmt;
+            st->recv[i].fn = fupg_recv_text;
         return;
     }