Added a ThenFunc, just to avoid http.HandlerFunc calls.

Thanks for alice =)
After converting a few projects over to using Alice I ended up with quite a few lines like:  

    chain.Then(http.HandlerFunc(someHandler))

I added a helper func, `ThenFunc(http.HandlerFunc)`, to shorten things up a bit:  

    chain.ThenFunc(someHandler)

Just keeps one from creating a wrapper, or always having to create an http.Handler.