Removed HTTP artifacts

...as they are now part of the phly/http package

- Renamed Middleware::handle() to Middleware::__invoke(), to make it compatible
  with the new phly/http implementation.
- Updated code and tests to refer to the `Phly\Http` and not
  `Phly\Conduit\Http`.
Updated README

... to remove API reference to `Phly\Conduit\Http`, as well as to reference
`Phly\Http` in all examples that reference request, response, and/or server
instances.

Additionally, updated the `composer require` statement to pull in the
`psr/http-message` and `phly/http` packages, as they are each at dev dependency.
Updated CHANGELOG

- Added notes about what has changed for 0.3.0
Merge branch 'feature/http-separation'

Refactored to use phly/http instead of a self-contained HTTP implementation.