`serif dev` server should serve up 404s

1. `serif new`
2. `serif generate`
3. `serif dev`
4. Visit http://localhost:8000/

The `serif dev` process gives a bunch of errors because `File.read(file)` was called but `file` was `nil`. This comes from the `/favicon.ico` request, and since the file doesn't exist, `File.read(file)` fails.

The fix is to define a `not_found` block and return `404` for the `get *` handler.