Separate observing of Note and MergeRequests

* Move is_assigned? and is_being_xx? methods to IssueCommonality

  This is behavior merge requests have in common with issues. Moved
  methods to IssueCommonality role. Put specs directly into
  merge_request_spec because setup differs for issues and MRs
  specifically in the "closed" factory to use.

* Add MergeRequestObserver. Parallels IssueObserver in almost every way.

  Ripe for refactoring.

* Rename MailerObserver to NoteObserver

  With merge request observing moved out of MailerObserver, all that
  was left was Note logic. Renamed to NoteObserver, added tests and
  updated application config for new observer names. Refactored
  NoteObserver to use the note's author and not rely on current_user.

* Set current_user for MergeRequestObserver

  IssueObserver and MergeRequestObserver are the only observers that
  need a reference to the current_user that they cannot look up on
  the objects they are observing.

spec/models/merge_request_spec.rb

@@ -50,4 +50,47 @@ describe MergeRequest do
       merge_request.mr_and_commit_notes.count.should == 2
     end
   end
+
+  subject { Factory.create(:merge_request) }
+
+  describe '#is_being_reassigned?' do
+    it 'returns true if the merge_request assignee has changed' do
+      subject.assignee = Factory(:user)
+      subject.is_being_reassigned?.should be_true
+    end
+    it 'returns false if the merge request assignee has not changed' do
+      subject.is_being_reassigned?.should be_false
+    end
+  end
+
+  describe '#is_being_closed?' do
+    it 'returns true if the closed attribute has changed and is now true' do
+      subject.closed = true
+      subject.is_being_closed?.should be_true
+    end
+    it 'returns false if the closed attribute has changed and is now false' do
+      merge_request = Factory.create(:closed_merge_request)
+      merge_request.closed = false
+      merge_request.is_being_closed?.should be_false
+    end
+    it 'returns false if the closed attribute has not changed' do
+      subject.is_being_closed?.should be_false
+    end
+  end
+
+
+  describe '#is_being_reopened?' do
+    it 'returns true if the closed attribute has changed and is now false' do
+      merge_request = Factory.create(:closed_merge_request)
+      merge_request.closed = false
+      merge_request.is_being_reopened?.should be_true
+    end
+    it 'returns false if the closed attribute has changed and is now true' do
+      subject.closed = true
+      subject.is_being_reopened?.should be_false
+    end
+    it 'returns false if the closed attribute has not changed' do
+      subject.is_being_reopened?.should be_false
+    end
+  end
 end