Fixing callables on model_mommy Recipe.

This commit addresses an issue with how callables were handled in model_mommy's
Recipe class. Previously, callables were detected and resolved on the `_make`
method of the Recipe class. This caused problems for things like the `_quantity`
api, since subsequent calls to mommy.make would just return the same value that
was first resolved in `_make`. If you tried to introduce randomness into your
mommy generations, you'd find this to be impossible via Recipe and _quantity.

We now pass callables straight through to `mommy.make`. There is new logic to
detect a callable and guarantee that is called each time make is called.

Additionally, a new test method has been introduced to ensure that the
`_quantity` is handled correctly.