I tried adding ClientFactory and WSRequestHolderBuilder, but WSAPI takes an implicit application and it's not very useful.  I think a magnet pattern might be useful here:

http://spray.io/blog/2012-12-13-the-magnet-pattern/

so you'd have different interfaces that could take `client(magnet:ClientMagnet)` or `url(magnet:WSRequestBuilderMagnet)` and then have `implicit client:WSClient` or `implicit app:Application` passed in.  I played with it, but trying to keep this PR small again :-/