Adds a Setting to control how an Issue's done_ratio is calculated:

* Issue field (default) - the done_ratio field for the Issue
* Issue status - uses the Issue Status's value

  #4274

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@3151 e93f8b46-1217-0410-a6f0-8f06a7374b81