Support for dropping outliers

[rubenlg]

One would think that the more samples we take, the more stable is the result. However, taking more samples also means there is a higher chance of getting an interference from the system (some daemon doing expensive work, flushing of caches, etc).

This feature request is about having a statistically rigorous way of dropping outliers before computing the confidence interval, so that one or two crazy measurements don't cause an "unsure" result, and adding more samples guarantees getting a more stable result.

This should be optional, not hard-coded, because outliers are not always independent from the page being tested (e.g. if a page has a 1% chance of hitting an expensive GC).