Make compatible with redmine 2.3

I may have not branched off the correct repository here, however, this update makes everything compatible with Redmine 2.2 and 2.3 (I have not tested 2.0 or 2.1).  Also, it makes it so that you don't have to do any manual work to get the open_flash_chart assets enabled as I registered open_flash_chart as a redmine plugin in the redmine_version_burndown init script.