Implement http_parser_pause().

Summary:
- Add http_parser_pause() API. A callback may invoke this at any time.
  This will cause http_parser_parse() to return indicating that it
  parsed less than the number of requested bytes and set an error to
  HBE_PAUSED. A paused parser with fail with HBE_PAUSED until it is
  un-paused with http_parser_pause().
- Stop using 'state', 'header_state', 'index', and 'nread' shadow
  variables and then updating their http_parser fields when we're done.
  Instead, update the live values as we go. This will make it possible
  to return from anywhere in the parser (say, due to EPAUSED) and have
  valid/expected state.
- Update state before making callbacks so that if the want to pause,
  we'll know the correct state already.
- Make sure that every callback has a state that uniquely identifies the
  next step so that we can resume in the right place if we were suppoed
  to be paused.
- Clean and re-factor up CALLBACK() macros.
- Use CALLBACK() macros for (almost) all callbacks; on_headers_complete
  is still a special case. This includes on_body which we used to invoke
  manually with a long run of bytes. We now use a 'body' mark and hit
  its callback just like every other data callback.
- Clean up (most) gotos and replace with real states.
- Add some unit tests.

Fixes #70