Custom consumer state check-pointing

[kevinballard]

• Add the ability to save a custom consumer state along with the log offset, allowing for stateful consumers to stop and resume gracefully
• Maintains the original API when using KinesisConsumer directly as an iterator
• Also emit shard_id with the "rich" generator
• Allow an optional consumer name so that state for multiple consumers of the same stream can exist in the same DynamoDB table
• Update README examples to Python 3