feat(tooltip): created initial version with accompanying demo and tests

This is in an initial state for everyone's review.

**Implemented:**

- Positioning (top, bottom, left, right)
- Transition (fade only, interim until $transitions)

**Outstanding:**

- Popup delay
- Additional triggers (e.g. focus/blur)
- HTML tooltip values
- Default visibility

These features will are fairly trivial to implement. I can do so this weekend if we're clear with the rest of this. But, there are two more outstanding features that will require additional discussion.

- Placement as a function: I don't think this makes sense for a directive
- Selector delegation: this is (potentially) more messy than with jQuery

Let me know what you think.