Interface cleanups before a 1.0 release

• Interface change: The Request and Response module types now explictly
  signal Eof and Invalid (for errors), to help the backend distinguish them.
• Interface change: Unify HTTP body handling across backends into a Cohttp.Body
  module. This is extended by Async/Lwt implementations with their specific
  ways of handling bodies (Pipes for Async, or Lwt_stream for Lwt).
• [lwt] Interface change: HTTP client calls now raise Lwt exceptions rather
  than return an option type. This permits better error handling in Lwt.
• [lwt] Interface change: The Server callback now always provides a body
  argument, since Cohttp_lwt_body now explicitly supports empty bodys.
• Add Cohttp.Header.is_keep_alive to test if a connection should be reused.
• [lwt] Respect the keep-alive header in the server request handling.
• [async] Add a Body that takes a Pipe or a string, similarly to Lwt.
• Install cohttp-server binary even if tests are disabled.
• Begin an examples directory with some simple uses of the library.