Don't use Express for lambdas

This would be a breaking change, but using `express` means routes aren't compatible with plain lambads (like `now`).

- Examples will need to use `res.end` and `res.writeHead`.
- Any complex examples/routes can still depend on `express` as a dependency.
- Params can be added to `req.query` via routing: https://spectrum.chat/zeit/now/are-url-parameters-possible-with-v2-routing~e1fec1df-09ff-4614-b031-4a0279b84bae