XD-176: Add exponential moving average support.

RichGauge now optionally supports as an alternative to the simple mean.
If the "alpha" smoothing constant is set for the gauge instance it will
calculate an exponential moving average instead.

The "mean" property of RichGauge has been renamed to "average" to
reflect this.
XD-176: Update toString method of RichGauge

Added the "alpha" value.