New feature: Sprint velocity calculator / chart

Need to develop a new module which will help the JA users to calculate the velocity for new sprint. This module will also include the velocity line chart which will display velocity for last x number of sprints providing the users to identify the performance of the team.

Based on the worklog added by users, this report will display the growth / downgrade in performance of the team.

Completion of #26 would be an added advantage for this module.

This module / feature would be implemented only based on popularity. If you would like this module be implemented then +1 this ticket.