feat(graphs.graph) First pass at highlighting on hover (ready)

- Highlights the nearest datapoint on <code>mouseover</code> and hides it on <code>mouseout</code>
- Expects data-points to be at same intervals.

![screen shot 2013-08-15 at 4 53 42 pm](https://f.cloud.github.com/assets/523368/972728/e0b7e8b0-0606-11e3-89ba-85eb3934b584.png)
![screen shot 2013-08-15 at 4 59 23 pm](https://f.cloud.github.com/assets/523368/972729/e2a67d76-0606-11e3-95a8-b5e517d91a0e.png)
![screen shot 2013-08-15 at 4 59 53 pm](https://f.cloud.github.com/assets/523368/972731/e51ce27a-0606-11e3-8f91-d7686f3078bd.png)