add select_dump method

[Jaymon]

this method will allow you to specify the table you want to dump by creating a select query, that way you can only easily backup just a subset of the rows

def select_dump(self, table, queries, outfile=None, pipes=None):
        '''
        this uses a flow discussed here (in order to output only certain rows)
        http://dbaspot.com/postgresql/348627-pg_dump-t-give-where-condition.html
        '''
        if not outfile: outfile = self._get_outfile(table)
        if isinstance(queries, basestring): queries = [queries]
        if not queries: raise ValueError('no queries to run')
        if not table: raise ValueError('no table')

        args = []
        args += [self.db]

        temp_table = 'temp__{}'.format(table)
        # could do print statements using \qecho
        # http://stackoverflow.com/questions/10663520
        run_queries = [
            'DROP TABLE IF EXISTS {}'.format(temp_table),
            'CREATE TABLE {} (LIKE {})'.format(temp_table, table),
        ]
        # add all the queries that were passed in so our temp table gets populated with the data we want to keep
        for q in queries: run_queries.append('INSERT INTO {} {}'.format(temp_table, q))

        self._run_queries(run_queries)

        if not pipes: pipes = []
        pipes.append('sed -e "s/{}/{}/g"'.format(temp_table, table))

        self.table_dump(table=temp_table, pipes=pipes, outfile=outfile)

        self._run_queries(['DROP TABLE {}'.format(temp_table)])
        return True