Implement scripting for KPI reports

KPI reports can be generated from script in yaml format. Scripting
provides a way to specify goals in human-readable form. Targets can be
grouped by modules, companies and users. Single target can be of following
types: to reach metric value, to reach position in top, to reach
minimum percentage value or to become core engineer. The example
is located in stackalytics/dashboard/templates/kpi/kpi_script.html

Change-Id: Idec4db58f5134d84009ba982121af3f086e4ad46