[enh] add safesearch to google_images

searx/engines/google_images.py

 # engine dependent config
 categories = ['images']
 paging = True
+safesearch = True
 
 # search-url
 url = 'https://ajax.googleapis.com/'
-search_url = url + 'ajax/services/search/images?v=1.0&start={offset}&rsz=large&safe=off&filter=off&{query}'
+search_url = url + 'ajax/services/search/images?v=1.0&start={offset}&rsz=large&safe={safesearch}&filter=off&{query}'
 
 
 # do search-request
 def request(query, params):
     offset = (params['pageno'] - 1) * 8
 
+    if params['safesearch'] == 2:
+        safesearch = 'on'
+    else:
+        safesearch = 'off'
+
     params['url'] = search_url.format(query=urlencode({'q': query}),
-                                      offset=offset)
+                                      offset=offset,
+                                      safesearch=safesearch)
 
     return params