decompression: Check for gzip using only first byte

Closes chfoo/wpull#155
.http.connection: Don't leak tracebacks across exception contexts

Python 3 seems to want to store the traceback on the exception object.

Side effect is that exceptions thrown by the client no longer contain
their tracebacks.
async: Ensure future is consumed after timeout.
Use manhole to create a Unix domain socket-accessible REPL

Connect to it with:

rlwrap nc -U /tmp/manhole-PID
Never fsync... who has time for that?
Specify tornado version in setup.py
Use a beautifulsoup4 compatible with Python 3.5