Examine page add affordance and tooltips to cols

Adds CSS tooltips to display the INFO attribute description and type from the VCF, as well as a slight underline as an unobtrusive affordance for the user.

![](http://cl.ly/image/2J3z0x3S3u1r/Screen%20Shot%202014-09-17%20at%207.04.12%20PM.png)

fixes #35 
How about `InfoColumnHeader`? Not sure I like tag names in component names -- feels like an implementation detail.