Examine: Improve Karyogram interactivity & info

Karyogram now has 1) clickable chromosome labels 2) expands to fit the
full width of the page 3) doesn't overflow its bounding box 4) is
colored by the number of variants in a given bin.

fixes #29 fixes #65 fixes #114 

![](http://cl.ly/image/1Y043B3d2103/Screen%20Shot%202014-10-27%20at%205.33.30%20PM.png)

(NB: still can zoom out infinitely; want to improve binning, make it more dynamic; doesn't resize when the window is)
Should this be a check on `props.position.chromosome`? I don't follow why the old value of `position` would be relevant here.