0.9.0

• Removed drain_events. Events are now drained by calling Client::close or on the
  transport on Transport::shutdown.
• Removed Hub::add_event_processor. This was replaced by Scope::add_event_processor
  which is easier to use (only returns factory function)/
• Added various new client configuration values.
• Unified option handling

This is likely to be the final API before 1.0