Allow for configurable batch size when sinking events to Kinesis

This updates the KinesisSink to use `putRecords` instead of `putRecord` for higher throughput.  When configuring the sink, the user can set the batchSize between 1 and 500 which will determine the max records that will be attempted in a single transaction.  Larger values should allow for greater throughput at the cost of latency.

I've also updated the README and cleaned up some of the other code for consistency (use of constants, whitespace, configuration checks).