Adds test_helper.rb template to the plugin generator.

Files:
modified: /branches/work/hooks/lib/generators/redmine_plugin/redmine_plugin_generator.rb (try)
added: /branches/work/hooks/lib/generators/redmine_plugin/templates/test_helper.rb (try)
modified: /branches/work/hooks/lib/generators/redmine_plugin_model/templates/unit_test.rb (try)
modified: /branches/work/hooks/lib/generators/redmine_plugin_controller/templates/functional_test.rb (try)