Emit response event to piped writeable streams

This allows for the writeable stream to get access to the
`http.IncomingMessage` object. This can for instance be used to access
the status code or the headers of the HTTP response.
Code cleanup
Update README.md to reflect changes in 329b15f
Merge pull request #3 from watson/expose-response

Emit response event to piped writeable streams
Handle back-pressure
Use simpler approach to guard against back-pressure

I'm not sure if the previous approach would call the `done` callback
twice if `req.write()` returned false. Also, there was a bug with
Node.js v0.11 where an error was thrown because of the callback given to
the `req.write()` method. I'm not sure if this was a bug in Node.js
v0.11.9 or my code, but this new code doesn't rely on the callback and
hence also works in Node.js v0.11.
Move examples to external modules
0.3.0
Update README.md
Add a flowHttp.statusCodes property