Bug fix on the video research method and coverage improvement.

The bug was solved by changing the maximum value of the 'search index'. Actually Youtube doesn't allow to get more than 500 results. That's why we changed the value of YOUTUBE_MAX_START_INDEX from 1000 to 500.

modules/youtube/test.py

@@ -42,7 +42,11 @@ class YoutubeTest(BackendTest):
         v = self.backend.get_video('http://youtu.be/UxxajLWwzqY')
         self.backend.fillobj(v, ('url',))
         assert len(v.url)
-        self.backend.browser.openurl(v.url)
+
+        try:
+            self.backend.browser.openurl(v.url)
+        except:
+            self.fail('can\'t open url')
 
     def test_weirdchars(self):
         v = self.backend.get_video('https://www.youtube.com/watch?v=BaW_jenozKc')