http.py: Relies on IOStream's closed state instead of ourself.
Implements --no-dns-cache.
Implements --accept/--reject.
Updates terse_options.txt
hook.py: Fixes handle_error AttributeError.
http.py: Don't raise exceptions if not active (chfoo/wpull#27).
changelog: Updates latest to 0.20.
Merge branch 'develop'
Bumps version to 0.20.