create & route 404 page

[skreutzberger]

optionally via a new/updated Abort Middleware which could later also be used to catch and log errors.

https://github.com/vapor/vapor/blob/master/Sources/Vapor/Middleware/AbortMiddleware.swift#L29

[skreutzberger]

add droplet instance to middleware after adding to droplet:

class MyMiddleware: Middleware {
    var drop: Droplet?
}

let mymiddleware = MyMiddleware()

let drop = Droplet(availableMiddleware: ["foo": mymiddleware])
mymiddleware.drop = drop

[skreutzberger]

additionally respect the requirements from https://github.com/vapor/vapor/issues/614