Releases/burnchart performance improvement

platform:  # supported: 2.2.4, 2.3.1
backlogs:  # supported: 1.0.1
ruby:  # supported: 1.9.3, 2.0.0

Release burnchart calculation has been split into two parts. The first part calculating individual stories is mostly unchanged, but result is now saved in a table with a rows per story per release per day. This allows the calculation of the burnchart to take advantage of SQL sum improving cached results by 3 orders of magnitude.
Initial calculation is still slow and adding new sprints to a release will trigger a complete rebuild so there's still room for improvement.

@backlogs/devs Could some of you find the time to review that I'm not messing up with the database migrations?

Thanks in advance,
Bo