Meter: EWMA deviates for low rates from the actual rate

The following example has the rate 15 values per minute, but it's calculated as 24 ![meter_on_15_bpm](https://f.cloud.github.com/assets/894252/791690/6e257da0-eb4c-11e2-9b5d-a70d1c50fd4f.png).

The proposed patch is going via making sure that EWMA intervals are strongly adjacent without time gaps.