New formula to calculate performance index

[supasate]

For each selected period,

Performance = A number of resolved pins in selected period / (A number of previous active pins + A number of new pins in selected period)

Ex. Current date is 15 Jan 2017 and we select 1 week period.

A number of resolved pins in selected period = All pins marked as resolved during 9 - 15 Jan.
A number of previous active pins = All pending + assigned + processing pins created before 9 Jan
A number of new pins in selected period = All pending + assigned + processing + resolved pins created since 9 Jan to 15 Jan

[supasate]

@parnurzeal API must support querying by resolved date.

[puripant]

NOTE: make sure that is_merged pins are not included in the formula