[PyGreSQL] Change to preferred startup method

[Charlie Root]

Up to now the README has said that this is the preferred method to use
the pg module.

------------------------- OLD -----------------------------------
from pg import DB
db = DB(...) # See description of the initialization method below.
-----------------------------------------------------------------

I am changing this to the following.

------------------------- NEW -----------------------------------
import pg
db = pg.DB(...) # See description of the initialization method below
-----------------------------------------------------------------

The reason for the change is that I have just realized that if you do it
the old way you don't have access to the error attribute.  Now you can
do something like this code.

    try: db.query("INSERT INTO ...")
	except pg.error: print "Insert failed"

So any syntax or name errors will dump a traceroute while an insert failure
(or any database error) can have an error message printed instead.