Moves Project#principals to a regular method.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11608 e93f8b46-1217-0410-a6f0-8f06a7374b81
Moves Project#users to a regular method.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11609 e93f8b46-1217-0410-a6f0-8f06a7374b81
Always instanciate a relation.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11610 e93f8b46-1217-0410-a6f0-8f06a7374b81