Resolve performance regression by not wrapping IntConstant data in Lazy wrapper.

By wrapping IntConstant raster data (used for no data tiles), the performance
benefit was being lost, as the lazy wrapper was generating an empty raster when
force() was invoked.
Remove println statement in ThoroughputLimitedReducer.
Clean up of TiledRasterData.
Move zonal summary operations to zonal.summary package.
Merge pull request #343 from joshmarcus/topic/zonal_summary

Resolve performance regression with zonal summary ops and change package.