Updated to psr/http-message 0.5.1 & phly/http 0.7.0

Updates Conduit to use psr/http-message 0.5.1 and phly/http 0.7.0. Changes
include:

- The HTTP decorators have been updated to follow
  `Psr\Http\Message\IncomingRequestInterface` changes and
  `Psr\Http\Message\OutgoingResponseInterface`. In some cases, that means the
  removal of setters, and changes to constructors.
- URLs are now handled internally by the HTTP messages as strings; whenver
  property access was required, the URI is either cast to a `Phly\Http\Uri`
  instance or `parse_url()` is used.
- Tests were updated to accommodate the changes to IncomingRequestInterface
  (primarily around the constructor).
Updated CHANGELOG
Merge branch 'feature/http-0.7.0'

Close #11