Merge static_world and web_server examples

The new example is called file_server and it's basically
the same as web_server was. The name is clearer than the
original, all examples being "Web servers".

The new example is also tested and the test suite has
been refactored a little.

examples/README.asciidoc

@@ -21,6 +21,9 @@
 * link:eventsource[]:
   eventsource emitter and consumer
 
+* link:file_server[]:
+  file server with directory listing
+
 * link:hello_world[]:
   simplest example application
 
@@ -42,15 +45,9 @@
 * link:ssl_hello_world[]:
   simplest SSL application
 
-* link:static_world[]:
-  static file handler
-
 * link:upload[]:
   multipart/form-data upload
 
-* link:web_server[]:
-  serve files with lists directory entries
-
 * link:websocket[]:
   websocket example