Init models in constructor with shells

This commit will initialize models set by `$uses` in the `__construct()` in shells. Currently you must call `initialize()` in order to use models in a shell.

I did not find any test cases nor tickets directly showing the current behavior is the desired behavior. The [docs](http://book.cakephp.org/2.0/en/console-and-shells.html#using-models-in-your-shells) indicate this change is the desired behavior. Thus I assume the current behavior is incorrect and this commit will fix it.