[stats] Enable performance endpoint sorted by runtime.

Creates an endpoint to visualize the slowest chronos jobs.

Allows users to see the (sorted) list of slowest chronos jobs by various distribution parameters:

You can sort by mean, median, 75thPercentile, 95thPercentile, 98thPercentile, and 99thPercentile by hitting the endpoint at /scheduler/stats/{whatever}

Returns a JSON of the form [ {"jobNameLabel": jobname, "time": timeInSeconds}, {"jobNameLabel": jobname, "time": timeInSeconds}, ...]

Also provides a clickable bar chart for easier visualization - currently we render the top ten jobs.

@brndnmtthws @hcai @andykram @krisr 

![barchart](https://f.cloud.github.com/assets/525770/1084471/55100bc2-15af-11e3-98eb-18df7d68f8a7.gif)