Support for X-Powered-By being user configurable, including disabling

This PR makes it possible to control the X-Powered-By header sent by go-json-rest applications. This includes disabling sending the header for those applications that may wish to (eg to save a tiny amount of bandwidth, or for [security reasons](http://serverfault.com/questions/395332/whats-the-use-of-x-powered-by-server-and-other-similar-http-headers)).

I have added a test that ensures the existing, default behaviour remains sending the header. There are additional tests which demonstrate changing the header or disabling it entirely.