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
remove superfluous grid line for log scale, refs #389
Merge pull request #867 from obfuscurity/backport_log_scale_labels_fix

Remove superfluous grid line for log scale
backport user graphs sorting fix, refs #209
Merge pull request #869 from obfuscurity/backport_sort_user_graphs

Backport user graphs sorting fix to 0.9.x
backport fix for reloading graphs tree in ajax callback, refs #603
Merge pull request #871 from obfuscurity/backport_reload_graphs_tree_after_ajax

Backport fix for reloading graphs tree in ajax callback to 0.9.x
backport fix for wrong series.name in percentileOfSeries, refs #501
Merge pull request #873 from obfuscurity/backport_fix_percentileOfSeries_name

Backport fix for wrong series.name in percentileOfSeries to 0.9.x
backport fix for dashboard initialState, refs #455
Merge pull request #874 from obfuscurity/backport_dashboard_initialState_fix

Backport fix for dashboard initialState to 0.9.x
backport margin() function to composer menu for 0.9.x, refs #343
Merge pull request #877 from obfuscurity/backport_margin_function_to_composer

Backport margin() function to composer menu for 0.9.x
fix rtd builds for 0.9.x
Merge pull request #879 from obfuscurity/fix_rtd_builds

Fix rtd builds for 0.9.x
backport minimumBelow() to 0.9.x, refs #878
Merge pull request #880 from obfuscurity/backport_minimumBelow

Backport minimumBelow() to 0.9.x