Merge branch 'master' into develop

Conflicts:
	node/connection.py
	node/market.py
	node/transport.py

* Conflicts in connection.py and transport.py due to
  listener refactor at #678. In order to ensure the instantiation
  of a single zmq Context, the initializers of the listeners have been
  expanded with an additional `ctx` argument.
* Conflicts in market.py due to style fixes in #658.