Change the event controller helpers for notes from path to url Closes #653

Currently certain note events, such as commenting on an issue, do not have a full URL in the RSS feed resulting in a broken link if you subscribe to the feed via email. Changing the event controller helpers for notes from path to url produces the desired result. Included a test to verify the change.

Related to issue #653 (closed).

One point in the code that I would like feedback on - on line 108 of events_helper.rb there is a function called "event_note_target_path". Should this be changed to "event_note_target_url" to more accurately reflect its return?