Hop-client

Changed
- The `hop.io.Producer` can be used to publish to more than one topic with a single instance.
To support this capability, the `write` method now accepts an optional `topic` argument to
direct to which topic to publish.
- The error message resulting when attempting to open a stream with ambiguous credentials has been
further clarified.

Added
- Users can now explicitly stop the `hop.io.Consumer`'s processing loop.
(Thanks to myNameIsPatrick)
- A message model for GCN text format notices has been added. The original notice text is preserved
unmodified, but parsing is performed to provide convenient programmatic access to the message
fields in memory.

Changed
- Create Blob objects instead of crashing on binary data when deserializing
- Fix handling of IPv6 addresses and default port numbers
- Fix authentication procedure in RobustPublisher

Added
- Allow messages to be consumed without deserialization
- Expose ability to commit offsets synchronously

Changed
- The command line tool now supports sending data which is piped to it either as JSON or as the raw
BLOB format.

Added
- Messages are now sent with UUIDs, attached as the `_id` header, and the username of the sender as
the `_sender` header.

Added
- Support for authentication using the "OAUTHBEARER" mechanism.
- An optional `timeout` parameter for `hop.io.list_topics` which can be used to be used to
prevent overly long or indefinite delays from this function when brokers are offline.

Changed
- All messages returned from `hop.io.Consumer` will now be instances of message model
classes. This means that for receiving unstructured JSON messages (now handled by
the `JSONBlob` class), users should retrieve the deserialized object(s) by accessing
the `content` property of the message, rather than expecting the message to directly
be the serialized object(s).
- `hop.io.Producer` will now use a Kafka message header to indicate the format used for
encoding each message it sends. `hop.io.Consumer` will read these headers, but also
has backwards compatibility with the JSON envelope format used in previous versions.

Added
- Support for sending messages encoded using the
[Avro container format](https://avro.apache.org/docs/current/spec.html#Object+Container+Files).
- Messages not formatted as JSON can now be safely accepted by `hop.io.Consumer`,
although it will not necessarily be able to provide useful deserialization of
unregistered formats. Messages which are not recognized will be returned as
instances of the `Blob` class, whose `content` property will contain the raw
bytes of the message data.