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
The Checkpoint functionality is an important part of the library and
previously it wasn't obvious that the Consumer was defaulting to Redis
for this functionality.
* Add Checkpoint as required param for new consumer
