Merged r4293 from trunk.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/branches/1.0-stable@4331 e93f8b46-1217-0410-a6f0-8f06a7374b81

lib/redmine/plugin.rb

@@ -90,6 +90,13 @@ module Redmine #:nodoc:
       @registered_plugins = {}
     end
 
+    # Checks if a plugin is installed
+    #
+    # @param [String] id name of the plugin
+    def self.installed?(id)
+      registered_plugins[id.to_sym].present?
+    end
+    
     def initialize(id)
       @id = id.to_sym
     end