Extend tooltip for drilldown link

It took me a bit to find out that variables need to be passed with var-variableName. 
The behavior is not specific to the drilldown link but I couldn't find it in any documentation elsewhere and it might help others to have it documented right here.
Added graphite function 'useSeriesAbove'
Add missing default parameters to useSeriesAbove
Merge pull request #1214 from DazWorrall/useseriesabove

Added graphite function 'useSeriesAbove'
Merge pull request #1210 from falkenbt/patch-1

Extend tooltip for drilldown link
Panel: drilldown link, moved new params tooltip to params property, #1210
Graph: fix for series tooltip when one series is hidden/disabled, #1199
Singlestat: small fix for threshold evalutations, use greater or equal for threshold checks, Fixes #1192
Small fix, removed test code change