Remove dependency on express/connect emitted event

Instead of listening for the `header` event, which is something
express/connect provides, hijack `writeHead` which gets called
implicitly (or explicitly, if the user wants) on every response.

This will allow people to use `client-sessions` outside of the 
context of an express/connect app (which is exactly what I'm
trying to do!)

Passed all tests locally. Sorry about that one whitespace diff

/cc @thisandagain