Translate statcmp.py to JavaScript

This PR translates [statcmp.py](https://github.com/mozilla/pdf.js/blob/master/test/stats/statcmp.py) to JavaScript (compare #4496).

The translation is close to 1:1, with the following minor exceptions:
* Differences are displayed with opposite signs. `-90ms` and `-90%` now means that `current` is *faster* than `baseline`. Since all numbers are in ms, I thought that this shows *lower is better* more clearly. It also makes more obvious that it means *10x faster* (compared to *1.9x faster*).
* If results are not grouped by 'stat', then only `Overall` results are used (`page request` and `rendering` are  dropped). I thought it is more intuitive to show the `Overall` score in this case, rather than the average of `Overall`, `rendering`, and `page request` (which is 2/3 of `Overall`). As a side effect, the significance test delivers different results in this case.
* The number of samples in each group is displayed (column `Count`).
* The javascript [ttest](https://www.npmjs.org/package/ttest) delivers slightly different results python's [ttest](http://docs.scipy.org/doc/scipy-0.13.0/reference/generated/scipy.stats.ttest_ind.html), although differences are not big.