Fixing tooltip for sparklines so that HTML works.

Currently, you get:

![current](http://f.cl.ly/items/2Z0r381a2s0n0H0f2P3B/Screen%20Shot%202013-07-09%20at%201.37.44%20AM.png)

Instead, what you want is:

![better](http://f.cl.ly/items/082q1k0X3T1F030P0v1b/Screen%20Shot%202013-07-09%20at%201.38.40%20AM.png)