Circumvent fixed position stacking issues by using transparent overlay

If you're interested (though changes visual behaviour).

- Make overlay transparent (use only to capture click events to exit)
- Make helper transparent (used to highlight target with box shadow)
- Remove any logic to reposition target or its parents. Assumes interactivity with target is undesirable whilst open.
- Position fixed helper if target or any of its parent are fixed.
- Remove button styles on assumption that apps will generally want to use global styles for buttons.

Worked really well for us with both static and fixed targets...

![image](https://cloud.githubusercontent.com/assets/939376/3616680/2ac5ab90-0dd6-11e4-8060-5ab2a99daaca.png)

![image](https://cloud.githubusercontent.com/assets/939376/3616685/32c2a8d4-0dd6-11e4-89cc-c061b84fe31a.png)