Added in argument for psycopg2.connect() and connection.__init__, and
for the connection.async attribute.

The behaviour of connect() is unchanged: either dsn or params must be
specified.

connect() docs updated to document the arguments merging.

This is equivalent to what proposed in #363, but with a much simpler
implementation.

Also expose the type from the extensions module, not from the main
module.

This is for people using dtuple.py; a dtuple.DatabaseTuple instance
keeps a reference to cursor.description, which is not picklable because
psycopg2 doesn't export the Column namedtuple it uses.

This commit exports the Column namedtuple, and includes a test to verify
the pickle/unpickle works after exporting Column.

They were only used to generate docs with Epydoc, now largely forgotten.

Imports in extras cleaned up to expose the API only.