Implement Conduit-specific HTTP functionality

Use decorators, instead of baking functionality directly into the phly/http
implementation. This includes:

- Request decorator handles arbitrary properties
- Response decorator adds in write/end/isComplete methods, and the functionality
  for ensuring immutability when complete.

Updated Next, Dispatch, and FinalHandler to expect the concrete decorators.
Middleware accepts any PSR versions, but decorates them at invocation.
Merge branch 'feature/http-decorators'

Close #3