move default-route logic to server-router

[yoshuawuyts]

The custom logic here is a bit hairy, we should move it to https://github.com/yoshuawuyts/server-router and move semver major on it. Would be a patch release here tho

[yoshuawuyts]

Oh, @timwis pointed out yesterday that we could also just use:

app.route('GET', '*', cb)

Which would catch all routes as a default. Can remove the default clause from both this and server-router I reckon. Yay