Releases/burndowncache

platform:  # supported: 2.0.4, 2.1.6, 2.2.3
backlogs:  # supported: 0.9.36
ruby:  # supported: 1.9.3, 1.8.7

this might not be the best solution, but eases the pain.
It does not optimize calculation time, though.

My first attempt to cache issue burndown data in the issues table itself would be even slightly faster, but i believe this one is architecturally the better approach.