Initial CRUD support for creating "Release Multiview" inspired by
http://blog.gdinwiddie.com/2012/12/04/multi-release-burnup/.

The idea is to put multiple releases into a burnup chart to get an
estimate of when a series of releases is finished according to the
velocity of all releases combined.

The user has to choose which releases to put into the multiview.
It might be interesting to see a series of sequential releases done
by the same team. Another possibility is to select parallel releases
from different teams if that makes sense in the given project.

As such the "Release Multiview" will just allow to group a selection
of releases visible in a given project's scope and generate a graph
with the combined release burnups.
Stacked release data calculation
Display release multiview graph
Added trendlines to multiview burnchart.
Enable basic test of burnchart view
Merge branch 'master' into releases/multi_release_view
Remove puts...
Sort releases by end date
Merge remote-tracking branch 'origin/master' into releases/multi_release_view
Merge branch 'master' into releases/multi_release_view
Correct sorting of releases in multiselect fields.
Fix not showing link to task board if sprint starts on current day
Merge pull request #995 from ande3577/bug/sprint_not_active_if_beginning_date

Fix not showing link to task board if sprint starts on current day
Refactor estimated series + show estimate end date along with releases when showing multiview.
Remove unused function
Merge branch 'master' into releases/multi_release_view
Update travis to RM 2.3.3