Rewrite the Gantt chart. #6276

Rewrite the Gantt chart. #6276

This version of the Gantt chart supports nested charts. So Projects,
Versions, and Issues will be nested underneath their parents correctly.

Additional features:

* Move all Gantt code to Redmine::Helpers::Gantt class instead of having it in
  the Gantt class, controller, and view
* Recursive and nest sub-projects
* Recursive and nest versions
* Recursive and nest issues
* Draw a line showing when a Project is active and it's progress
* Draw a line showing when a Version is active and it's progress
* Show a version's % complete
* Change the color of Projects, Versions, and Issues if they are late or
  behind schedule
* Added Project#start_date and #due_date
* Added Project#completed_percent
* Use a mini-gravatar on the Gantt chart
* Added tests for the Gantt rendering

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@4072 e93f8b46-1217-0410-a6f0-8f06a7374b81