Set defaults for host, port, dbname and user later in the invocation

[waterkip]

The problem is that if one uses sqlalchemy or some other tool that uses some
kind of object that returns None for a value, eg the port you need to
reimplement a lot of the code that is in this module already.

from sqlalchemy.engine.url import make_url

uri = make_url('postgres://localhost/foo');
# yields an error that a NoneType cannot be used for int();
password = getpass(uri.host, uri.port, uri.username, uri.database);

In order to fix this a lot of the defaults need to be implemented at the caller
level.

Furthermore, the default database name is the name of the user that is supplied
to the function, eg, a user 'foo' will default to the database 'foo', as seen
in behaviour by psql -U foo where 'foo' isn't your username of the OS.

Signed-off-by: Wesley Schwengle wesley@opperschaap.net