add "prefix()" select construct

[Mike Bayer]

for custom keywords at the front of a select:

    select([table.c.col1, table.c.col2](prefix("SQL_CALC_FOUND_ROWS"),))

produces:

   SELECT SQL_CALC_FOUND_ROWS table.col1, table.col2 from table

[Mike Bayer]

implemented this as

select().prefix_with('foo').prefix_with('bar')

changeset:3053

[Mike Bayer]

Removing milestone: 0.4.0 (automated comment)