optionally persist 0 values

When gographite is used to provide source side aggregation of data on multiple hosts, and the dataset is set to be be averaged by graphite between granularities, it's would be helpful for gographite to keep sending zero's instead of only sending non-zero values.

For example:

If gographite (with --flushInterval=60) on two separate minutes gets a value of 10, when it performs aggregate merging it doesn't count nulls at 0. This means a sumSeries query at a larger granularity will sum average per-minute value of 10, but queries at lower timeframes will get full resolution data and will sumSeries the correct values.