As we start to add more targets, we'll want a common way to parse output from taps. This adds a singer.parse_message function that takes a JSON line and returns an object that represents the message.

• Add three new namedtuple classes to represent message data.
  • RecordMessage
  • SchemaMessage
  • StateMessage
• Convert the write_record, write_schema, and write_state functions to use those classes.
• Add a parse_message function that takes a string and returns an instance of one of the message classes.
• Add unit tests