Adding an informative object to the connection

Hi,

a few days ago we were contributed a merge request for an `.host`
attribute on the connection. But looking back at the bug
(https://github.com/psycopg/psycopg2/issues/726) I had already noticed
there are other interesting informative connection functions in the
libpq. Some of them are already exposed as methods of the connection
class (e.g. get_backend_pid(), get_transaction_status() etc), but I
can see a trickle of requests for others, so I'd rather expose
most/all of them.

Instead of bloating the connection class with more and more attributes
and methods, I'm rather adding a connection.info method to retrieve
pgconn-specific information. The work is happening in this branch:

https://github.com/psycopg/psycopg2/commits/connection-info

I would softly deprecate the currently exposed informative methods:
not sending them away in a short time but giving a preference to use
from the 'info' object what belongs to the pgconn/libpq and to leave
on the connection things regarding the Python layer, and clean up the
interfaces a little bit.

Any input is welcome. Cheers!

-- Daniele