Add interval flush for DDB checkpoint
* Allow checkpointing on a specified interval
* Add shutdown method to checkpoint to force flush
Minor changes:
* Swap order of input params for checkpoint (app, table)
Addresses: https://github.com/harlow/kinesis-consumer/issues/39
Testing the components of the consumer where proving difficult because
the consumer code was so tightly coupled with the Kinesis client
library.
* Extract the concept of Client interface
* Create default client w/ kinesis connection
* Test with fake client to avoid round trip to kinesis
