Delay a reply

Added a delay function which causes the request to halt for N ms before replying. It is implemented by replacing the reply body with a stream which emits the original body after the timeout.

I believe it supports all of the possible reply methods, including replyWithFile and reply callbacks. If the reply callback returns a stream it will output the contents of that stream.

Includes tests and docs for the new functionality. (Sorry for the whitespace alterations.)

Closes #128