Convert Notice::asAtomEntry() to use Notice::asActivity() and Activity::asString()

We had two ways to generate an activity entry from a notice; one through
Notice::asAtomEntry() and one through Notice::asActivity() and
Activity::asString(). The code paths had already diverged somewhat. I
took the conditions that were in Notice::asAtomEntry() and made sure
they were replicated in the other two functions. Then, I rewrote
Notice::asAtomEntry() to use the other two functions instead.

This change passes the ActivityGenerationTests unit tests, but there
may be some other stuff that's not getting covered.

lib/activitysource.php

@@ -0,0 +1,56 @@
+<?php
+/**
+ * StatusNet, the distributed open-source microblogging tool
+ *
+ * Activity source, to save in <atom:source>
+ *
+ * PHP version 5
+ *
+ * LICENCE: This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * @category  Feed
+ * @package   StatusNet
+ * @author    Evan Prodromou <evan@status.net>
+ * @author    Zach Copley <zach@status.net>
+ * @copyright 2010 StatusNet, Inc.
+ * @license   http://www.fsf.org/licensing/licenses/agpl-3.0.html AGPLv3
+ * @link      http://status.net/
+ */
+
+if (!defined('STATUSNET')) {
+    exit(1);
+}
+
+/**
+ * Feed data to store in the <atom:source> element
+ *
+ * I wanted to use Atom10Feed but it seems more heavyweight than what's
+ * needed here.
+ * 
+ * @category  OStatus
+ * @package   StatusNet
+ * @author    Evan Prodromou <evan@status.net>
+ * @copyright 2010 StatusNet, Inc.
+ * @license   http://www.fsf.org/licensing/licenses/agpl-3.0.html AGPLv3
+ * @link      http://status.net/
+ */
+
+class ActivitySource
+{
+    public $id;
+    public $title;
+    public $icon;
+    public $updated;
+    public $links;
+}