requestsStats: add a unit test
Unit tests: fix assertMetric()
requestsStats: add latency related metrics (#134)

* smallestLatency: the time to the first byte of the fastest response
* biggestLatency: the time to the first byte of the slowest response
* medianLatency: median value of time to the first byte for all responses
Update unit test
Update README
Add fast-stats dependency for Travis CI
Merge pull request #146 from macbre/response-latency

Add latency related metrics