Adds a 'Add subprojects' permission.

* 'Add project' permission will let user create a root project
* 'Add subprojects' permission will let project members create subprojects

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@3238 e93f8b46-1217-0410-a6f0-8f06a7374b81