organize examples

This commit is largely based on work by Thiago Arrais (@thiagoarrais)
and Shane Bradfield (@l33tLumberjack). I wasn't really able to get their
PR reasonably merged onto the latest master, so I reapplied their changes
manually here and did some further improvements on that.

examples/complex/stream_modify.py

@@ -0,0 +1,20 @@
+"""
+This inline script modifies a streamed response.
+If you do not need streaming, see the modify_response_body example.
+Be aware that content replacement isn't trivial:
+    - If the transfer encoding isn't chunked, you cannot simply change the content length.
+    - If you want to replace all occurences of "foobar", make sure to catch the cases
+      where one chunk ends with [...]foo" and the next starts with "bar[...].
+"""
+
+
+def modify(chunks):
+    """
+    chunks is a generator that can be used to iterate over all chunks.
+    """
+    for chunk in chunks:
+        yield chunk.replace("foo", "bar")
+
+
+def responseheaders(flow):
+    flow.response.stream = modify