fixed carbon setup.py missing maintenance plugins

https://github.com/graphite-project/carbon/issues/106
Aggregate only if the percentage of points with values is greater than the xFilesFactor.

adresses https://github.com/graphite-project/carbon/issues/52 

and partially https://github.com/graphite-project/ceres/issues/6
Merge pull request #107 from anatolijd/megacarbon

ceres-maintenance rollup plugin fix