Version v0.3.9: fixes compatibility of doctests with rust 1.26.0

Version v0.3.8: added byte timeout

Version v0.3.7: fix timeout in websockets when ping_interval > inacti…

…vity_timout

Version v0.3.6: adds `add_date` method to server, and some error intr…

…ospection to client

Version v0.3.5: a bugfix release

Version v0.3.4: bugfixes in client state machine

Version v0.3.3: exposes low-level helpers for websocket codecs

Version v0.3.2: fix wakeup notification bug in client

Version v0.3.1: adds tools for pushback inside single request/response

Version v0.3.0: finally sends ping over websocket connections

This version introduces `Handle` argument to Loop constructors, that's only
imcompatible change.