Feature/opfquery

Okay so here's something akin to a wrapper API for doing a little more advanced queries without having to write actual SQL.

I have yet to hook it up to the models, but it should be straight forward.

Example query:

```objective-c

[[[[OPFComment query] column: @"post_id" is: @"2"] column: @"text" like: @"%%CSS%%"] column: @"user_id" in: @[@"1", @"2", @"3"]]

```
This will produce the following SQL:

    "SELECT 'comments'.* FROM 'comments' WHERE ('comments'.'post_id' = 2 AND ('comments'.'text' LIKE '%CSS%' AND 'comments'.'user_id' IN (1,2,3)))"