Handle transfer-encoding:chunked; misc. replay bugs.

Thanks for launching this! I just spent some time hacking on it, and once I got past beating my head on bizarre errors caused by gzipped transfer-encoding:chunked data, it worked really well (replayed 14/14 of my test cases and looked great).

In this pull req:

- Add a ChunkedLineReader to deal with replays with the
transfer-encoding: chunked header.
- Catch UnicodeDecodeErrors caused by multibyte characters getting
split during buffering.
- A couple of tiny bugs in replay.py