added __slots__ to decrease memory footprint of Link objects

--HG--
extra : convert_revision : svn%3Ab85faa78-f9eb-468e-a121-7cced6da292c%40413

scrapy/trunk/scrapy/link/__init__.py

@@ -77,6 +77,9 @@ class Link(object):
     Link objects represent an extracted link by the LinkExtractor.
     At the moment, it contains just the url and link text.
     """
+
+    __slots__ = 'url', 'text'
+
     def __init__(self, url, text=''):
         self.url = url
         self.text = text