Add 500px and Searchcode engines

Allow to search for images on 500px. It doesn't use the official API, but the page result. Less stable, but less API key to possess...

Two engines were necessary for Searchcode because there are to search mode : search for documentation or search for code example. Both use open APIs.

searx/settings.yml

@@ -64,6 +64,10 @@ engines:
 #    engine : filecrop
 #    categories : files
 #    shortcut : fc
+    
+  - name : 500px
+    engine : 500px
+    shortcut : px
 
   - name : flickr
     engine : flickr
@@ -114,6 +118,14 @@ engines:
   - name : stackoverflow
     engine : stackoverflow
     shortcut : st
+    
+  - name : searchcode doc
+    engine : searchcode_doc
+    shortcut : scd
+    
+  - name : searchcode code
+    engine : searchcode_code
+    shortcut : scc
 
   - name : startpage
     engine : startpage