add open method mixin for Postgres Database object

class Postgres_Mixin():

    """Serve common connection method for postgres."""

    def open(self, search_paths='public'):

    def open(self, search_path='public'):

        """ Explicitly open the database connection

        :note: This will send email to Database.CONNECTION_ERR_EMAIL_RECPS if

               unable to establish connection

        :param search_path: the search path to default to

        :return: True if connection established, else false

        """