feat(ngIndeterminate): indeterminate boolean attr

This PR adds a new `ngIndeterminate` directive that allows the user to set the `indeterminate` boolean attribute on checkboxes.
Here's the demo with checkbox on:

![screen shot 2014-08-15 at 5 33 55 pm](https://cloud.githubusercontent.com/assets/1271410/3940345/169f30d2-24dd-11e4-9fdb-212a9bb59bf6.png)

and with the checkbox off:

![screen shot 2014-08-15 at 5 34 04 pm](https://cloud.githubusercontent.com/assets/1271410/3940347/1ffd977c-24dd-11e4-841e-56b058424f5a.png)

Indeterminate is totally a thing, and we need this change because my team is manipulating DOM in controllers and I need to make them stop. :)