Redmine

This patch brings HowTo_Install_Redmine_in_a_sub-URI up to Rails 3.
It seems that the routes are loaded before config/environment.rb, so putting the Redmine::Utils::relative_url_root there does not work because the scope of the routes has already been set. Each plugin needs to be modified with just the scope.

Tested on:

Environment:
  Redmine version                2.3.3.stable.12244
  Ruby version                   2.0.0-p247 (2013-06-27) [i686-linux]
  Rails version                  3.2.13
  Environment                    production
  Database adapter               Mysql2
Redmine plugins:
  redmine_inline_attach_screenshot 0.4.6
  redmine_stealth                0.6.0

For the record, both plugins work fine with almost no modification.