3.0 - Only the primary query should be marked as such.

This makes callbacks like ```beforeFind()``` receive the ```$primary``` parameter as expected:

The query for the primary model gets ```$primary = true``` and all subsequent queries will have the value set as ```$primary = false```.

My apologies, but I don't know how to test this without incurring in constructing tables with associations, and setting ```beforeFind()``` callbacks to every table, and later testing ```$primary``` inside every one of them. There should be an easier way I believe.