Prevents tooltip from blocking other interactions

Adds `pointer-events: none` to the tooltip to prevent the tooltip from blocking other interactions, as seen here: 

__Original behavior (can't drag slider due to tooltip from slider below)__
![tooltip](https://cloud.githubusercontent.com/assets/848347/5574442/215fb864-8f74-11e4-9153-c2d5783c9b30.gif)
__New behavior (tooltip doesn't appear until hovering over slider, solving issue)__
![tooltip2](https://cloud.githubusercontent.com/assets/848347/5574469/ab95e3be-8f74-11e4-9ddd-d90a847d78d0.gif)