BinderD and auto-updating logs support

[andrewosh]

This PR adds BinderD, a background process running on the API server that makes it easy to create/manage long-running processes for log support, cluster monitoring, inactive app deletion, etc. All BinderD modules (BinderDModule base class) can receive messages over 0MQ using the BinderClient class. A BinderClient is passed a module name as a parameter, and exposes a simple interface on top of 0MQ's send and recv messages.

Currently, the only BinderD modules available are the log_reader and the log_writer modules.

• log_reader responds to log queries (type get), and supports the optional since parameter for fetching all logs matching a filter generated since the specified timestamps.
• log_writer takes messages (type log), each containing required tags and levels, and:
  1. records those messages in the appropriate log file
  2. publishes those messages (using 0MQ's Pub/Sub features) with an app name (root if the name's excluded) as the topic.

Additionally, this PR includes a new WebSocket endpoint, which queries the log_reader module and subscribes to all messages from the log_writer's subscriber socket.

All print statements in Binder have been replaced by the appropriate log_* messages in log.py, which send messages to the log_writer module.