FEATURE/BUGFIX: properly support HEAD requests

First, enable specific routing for Head requests via router.Head(...). I don't anticipate this to be commonly used but seems good for completeness.

More importantly, if a HEAD request comes in, and there's no route (see above), then try a GET route instead. It turns out (I believe) that Go's
default ResponseWriter will automatically 'discard' the body of the request for HEAD requsets, so there's nothing (besides routing) that I need
to do here.

Finally, restrict StaticMiddleware to only handling GET and HEAD requests. Eg, POST'ing to an image will return 404 now.