Add support for join with subselect

[nt-gt]

This is a split of #995 concerning the missing support for JOIN'ing with a subselect a la LEFT OUTER JOIN (SELECT ... ) AS alias ON alias.X = other_table.Y.

This is currently not possible with the SelectBuilder, which assumes that the joins will be tables.

[nt-gt]

@schauder Thanks for implementing this. :)