This update will allow Rails Engines to be installed as gems.  For example:

    # config/environment.rb
    Rails::Initializer.run do |config|
      ...
      config.gem 'timesheet_plugin'
    end

Will load the timesheet_plugins's RubyGem and allow it to work as if it was
installed to vendor/plugins.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@2904 e93f8b46-1217-0410-a6f0-8f06a7374b81