Add a report for spikes in the past N days

[jdm]

We should add a link to http://build.servo.org/intermittent-failure-tracker/search that reports any tests that had a spike in failure rates in the past N days. If the failure rate for a test is defined as the number of occurrences during a period P divided by the length of period P (ie. the daily average), then a spike occurs when the failure rate for period P is >= 1.5 * the failure rate for period P - 14. We can tweak the numbers to explore better calculations, but the point is that we would like to notice when a particular test suddenly starts failing at a much higher rate than it used to.