Add JavaScript and StringIO backends, and numerous interface improvements

Compatibility breaking interface changes:

• Rename Cohttp.Auth.t to Cohttp.Auth.credential and Cohttp.Auth.req
  to Cohttp.Auth.challenge. Also expose an Other variant
  to make it more extensible for unknown authentication types. The
  Cohttp.Auth functions using these types have also been renamed accordingly.
• Rename Cohttp.Transfer.encoding_to_string to string_of_encoding
  for consistency with the rest of Cohttp's APIs.
• The has_body function in the Request and Response modules now
  explicitly signals when the body size is unknown.
• Move all the module type signatures into Cohttp.S.
• If users have percent-encoded file names, their resolution is changed:
  resolve_local_file in Cohttp_async and Cohttp_lwt now always
  percent-decode paths (#157)
• Remove the Cohttp_lwt.Server.server type synonym to t.
• When reading data from a HTTP body stream using the Fixed encoding,
  we need to maintain state (bytes remaining) so we know when to finish.
  The Cohttp.Request and Cohttp.Response interfaces now expose a
  reader and writer types to track this safely.
• Add is_empty function to the Cohttp.S.Body module type.
• Add Strings representation to Cohttp.Body to efficiently hold a
  list of body chunks.
• Move flushing logic for HTTP bodies into the portable Request and
  Response modules instead of individual Lwt and Async backends.
• Port module interfaces to the latest Conduit (0.6.0+) API.
• Cohttp requires OCaml 4.01.0 or higher now.

New features and bugfixes:

• Add a Cohttp_lwt_xhr JavaScript backend that enables Cohttp logic to be
  mapped to XMLHTTPRequest in browsers via js_of_ocaml (via Andy Ray).
• Add a Cohttp.String_io and String_io_lwt module that uses OCaml
  string or Buffer.t to read and write HTTP requests and responses
  instead of network connections.
• cohttp_server_lwt and cohttp_server_async now return better errors (#158)
• cohttp_server_lwt and cohttp_server_async now serve indexes directly (#162)
• [lwt] Add stop thread to terminate a running server if it finishes (#147).
• Add Cohttp.Connection.compare to make ordering of connections possible.
• Add Body.map and Body.as_pipe to work with HTTP bodies more easily.
• Remove link-time dependency on camlp4 via META fixes (#127).
• Support HTTP methods and versions other than the standard ones. (#142).
• Improve cohttp_server_lwt and cohttp_server_async directory listings (#158)
• Fix Cohttp_async.resolve_local_file directory traversal vulnerability (#158)
• [async] In the Async server, do not close the Reader too early.
• [async] Close file descriptors more eagerly in the HTTP client (#167).
• Reduce thread allocation by replacing return <const> with return_none,
  return_unit or return_nil.