Add new "changed" function

This function checks if a metric has changed and returns 1 when it changed else 0
Rework of 'query-bulk' implementation for 0.9.x

Please see issue #378, I just reworked for latest 0.9.x branch and made configurable. Corresponding change for carbon was merged by @mleinart while ago - https://github.com/graphite-project/carbon/pull/132
Replacing #824.
Merge pull request #850 from deniszh/0.9.x_query_bulk

Rework of 'query-bulk' implementation for 0.9.x
Merge pull request #530 from criteo/changed-function

Add new "changed" function
Merge branch '0.9.x-upstream' into 0.9.x

Conflicts:
	webapp/graphite/render/datalib.py