add TAIL_INTERVAL configuration setting to use in tail_avg

[mabrek]

Checking last N datapoints gives different results on metrics with different resolutions. If anomaly is detected on 1 last datapoint or even 3 last datapoints in a metric with 2 seconds resolution that anomaly might disappear in 10 seconds. If metric has resolution of 5 minutes then there is quite a lot of time for human to notice detected anomaly.
Metrics with a different resolutions might be present in the same environment so single size (like 3 used in tail_avg) won't fit them all.
New configuration setting (TAIL_INTERVAL measured in seconds) needs to be added so that algorithms would compare datapoints from the last TAIL_INTERVAL seconds with the rest.

See the discussion at #43