HTML in tooltips and style tweaks

This allows us to optionally specify the #id of a node to place inside a tooltip via `data-tooltip-content`.

```
<p>
   Lorem <span data-tooltip data-tooltip-content="something">ipsum</span>
</p>
<div id="something">
   Hi!
</div>
```

Also included are some minor style tweaks. Adjusted default bg color to n-1, and spacing to something that works for both single text line and multi-line (list) content. 

![image](https://cloud.githubusercontent.com/assets/131831/4245269/c9c93a48-3a33-11e4-97e2-dc25dfcbc8db.png)

![image](https://cloud.githubusercontent.com/assets/131831/4245279/1710dbd0-3a34-11e4-9c3f-4983cfeaee0e.png)

Chucked in a .tooltip--spaceless variation that removes tooltip padding entirely if desired.

Existing title only tooltip functionality remains unchanged.
I would drop the assumption of `['data-hidden']` in favour of `show()/hide()`