Introduce a filtered flow writer, and use it in dump.py

Fixes #104

examples/flowbasic

@@ -3,8 +3,8 @@
     This example shows how to build a proxy based on mitmproxy's Flow
     primitives.
 
-    Note that request and response messages are not automatically acked, so we
-    need to implement handlers to do this.
+    Note that request and response messages are not automatically replied to,
+    so we need to implement handlers to do this.
 """
 import os
 from libmproxy import proxy, flow
@@ -19,13 +19,13 @@ class MyMaster(flow.FlowMaster):
     def handle_request(self, r):
         f = flow.FlowMaster.handle_request(self, r)
         if f:
-            r._ack()
+            r.reply()
         return f
 
     def handle_response(self, r):
         f = flow.FlowMaster.handle_response(self, r)
         if f:
-            r._ack()
+            r.reply()
         print f
         return f