subqueries with db builder

[Nodren]

i think the db builder is great, and especially with the rewrite it can be a whole lot better.

i'm thinking the value of a in, where, etc functions can be a db builder query itself... to allow for sub queries

an example, expressing this query:

select * from table 1
where id in (
  select table1_id 
  from table2 
  where something = 'yes'
)

would be expressed

$db->from('table1')->in($db->select('table1_id')->from('table2')->where(array('something'=>'yes')))

just an idea.

[Shadowhand]

This will be part of #204.