Rapid Board to handle release planning.

As seen in our documentation, in GreenHopper Classic mode one can handle a hierarchy of fixVersions and charts based on them (since the essence of the Sprint is bound to the fixVersion field). This is a good enough tool to plan releases, see the trend of burning rate, able to predict a release date.

However, in the GreenHopper Rapid Board, all work is organized in Sprints, completely independent from the Project Version.

The current workaround is to create independent Rapid Boards. However, it would be nice if this could be an embedded feature in GreenHopper next versions.