Release graphs burnup chart

Release graph has been made as a burnup chart for now. It can be enabled from the plugin settings.
It includes the following:
* Stacked bar graph showing closed, added and total points in a given release.
* Trend line estimating future work
* Planned velocity (basically setting a desired points/month and showing a trend line)
* CSV export of release graph data should be fixed
* Ability to adjust an issues relationship with a release (initial,continued,added,auto). Explanation will be added to the documentation. 

The burnup chart is by purpose optional at the moment as the performance is not satisfying for big projects currently. The data should now be aligned to make it easier to make a more fine grained cache solution. More on that later on the forum.