gh-9 : Adds support for back-fetching of older data in movingAverage

Although this adds support for back-fetching it removed the ability
to definie an arbitary number of timeSteps by integer.

If a moving average smaller than the timestep (or zero seconds etc)
the original metric is returned unchanged.
Merge pull request #16 from ciaranj/ash-movingAverage

gh-9 : Adds support for back-fetching of older data in movingAverage