move atom category to its own file, too

lib/activity.php

@@ -362,48 +362,3 @@ class Activity
     }
 }
 
-class AtomCategory
-{
-    public $term;
-    public $scheme;
-    public $label;
-
-    function __construct($element=null)
-    {
-        if ($element && $element->attributes) {
-            $this->term = $this->extract($element, 'term');
-            $this->scheme = $this->extract($element, 'scheme');
-            $this->label = $this->extract($element, 'label');
-        }
-    }
-
-    protected function extract($element, $attrib)
-    {
-        $node = $element->attributes->getNamedItemNS(Activity::ATOM, $attrib);
-        if ($node) {
-            return trim($node->textContent);
-        }
-        $node = $element->attributes->getNamedItem($attrib);
-        if ($node) {
-            return trim($node->textContent);
-        }
-        return null;
-    }
-
-    function asString()
-    {
-        $attribs = array();
-        if ($this->term !== null) {
-            $attribs['term'] = $this->term;
-        }
-        if ($this->scheme !== null) {
-            $attribs['scheme'] = $this->scheme;
-        }
-        if ($this->label !== null) {
-            $attribs['label'] = $this->label;
-        }
-        $xs = new XMLStringer();
-        $xs->element('category', $attribs);
-        return $xs->asString();
-    }
-}