Rework Flickr Engine

Everything was redone to use the API. It needs an API key, but it's worth it.
Everything works. Title, Image, Content, URL
The API allow lots of things. Thumbnails and date will be easy to add when it will be implemented in Searx.

Fix asciimoo/searx#126

searx/settings.yml

@@ -65,11 +65,12 @@ engines:
 #    categories : files
 #    shortcut : fc
 
-  - name : flickr
-    engine : flickr
-    categories : images
-    shortcut : fl
-    timeout: 3.0
+# api-key required: https://www.flickr.com/services/apps/create/
+#  - name : flickr
+#    engine : flickr
+#    categories : images
+#    shortcut : fl
+#    api_key: 'apikey' # required!
 
   - name : general-file
     engine : generalfile