Refactor to use phly/http

Now that I've separated the HTTP functionality into its own repo ([phly/http](https://github.com/phly/http), this pull request updates Conduit to use that package as a dependency, and to simply build on top of it.

The following changes needed to be made:

- Middleware::handle was renamed to Middleware::__invoke to make it compatible with `Phly\Http\Server`.
- All references to the Http subnamespace were updated to refer to the new package's namespace.
- The README file was extensively updated to remove the HTTP artifacts, as well as to ensure examples refer to the appropriate classes.