EXISTS / NOT EXISTS clause support for  DB_query_builder.

I need a query contains EXISTS ()  to check permission or non-completed relations etc.  So , I have just added EXISTS support to query_builder to be able to use as ; 

```
->order_by('post_date','DESC')
->where('EXISTS','(SELECT * FROM blog_privacy WHERE blog_privacy.post_id = blogs.post_id)',false)
```