opStrategy and ordering for ArithmeticPostAgg

Fixes #510
truncate segmentGranularity
Merge branch 'arithmetic-op-strategies' of https://github.com/metamx/druid into handle-arithmetic-ex
Revert "Merge branch 'arithmetic-op-strategies' of https://github.com/metamx/druid into handle-arithmetic-ex"

This reverts commit 5245422a62a2cccb27fef1d719c62bc2126ba5b7, reversing
changes made to fe03476c53cd3e8948368097394cfb1cd75e4e30.
Merge pull request #693 from metamx/handle-arithmetic-ex

Handle arithmetic ex - additional commit for PR #692