New IssueObserver class and spec.

Handles emails for new issues and reassigned issues.
Need to add creating a Note on Issue close.

app/models/issue_observer.rb

@@ -0,0 +1,17 @@
+class IssueObserver < ActiveRecord::Observer
+  cattr_accessor :current_user
+
+  def after_create(issue)
+    Notify.new_issue_email(issue.id) if issue.assignee != current_user
+  end
+
+  def after_change(issue)
+    if issue.assignee_id_changed?
+      recipient_ids = [issue.assignee_id, issue.assignee_id_was].keep_if {|id| id != current_user.id }
+
+      recipient_ids.each do |recipient_id|
+        Notify.reassigned_issue_email(recipient_id, issue.id, issue.assignee_id_was)
+      end
+    end
+  end
+end