Defined releases with bohansen's new graph

It is still work in progress, but i'll offer the merge anyways.
The resulting merge is mergeable into master (based on v0.9.32-24-g835b8d6).

bohansen_release_chart is merged in here, but some changes where required to make it work, most notably calculating based on new rb_issue_history.

This branch works on 'defined releases', thus not depending on sprint dates but on issue-release relationship.
The RbRelease object is shareable like a version, but is not a version object. It integrates into std. redmine views, filters, queries and bulk editing. It also extends the master backlogs view.

There are 2 new featuretests: releases3.feature testing the redmine integration and release_burndown.feature testing my current understanding of release burndowns.

The release.feature is still there but not worked out yet. These tests might be combined before going master.
Burndown calculations are not finished, some edge cases and ideas are still open todo.
Some cleaning up is still to do.