Wrap PQnotifies per GVL.

PQnotifies doesn't do/wait for network communication, but PQnotifies can deliver
notifications per callback. The registered callback function is then called with GVL locked.
This results in a rb_bug("rb_thread_call_with_gvl: called by a thread which has GVL.")
if we don't wrap PQnotifies likewise in order to unlock the GVL.
Merge pull request #10 from larskanis/wrap-pqnotifies-per-gvl

Wrap PQnotifies per GVL.