lazy import for uuid module

Attached patch moves uuid import from inside try-except
to register_uuid function.  Reason: uuid module import is *very*
heavy.  It goes into OS searching for various .dll/.so libraries,
lauches 'ldconfig' and so on...

With this patch, 200x python -c 'import psycopg2.extras'
goes from 22s to 7s.  (plain 'import psycopg2' is 6s)

--
marko