Add support to "AT_TIMESTAMP" ShardIteratorType

[M0dM]

As documented here:
https://boto3.amazonaws.com/v1/documentation/api/latest/reference/services/kinesis.html#Kinesis.Client.get_shard_iterator

I think those lines may be enough to support AT_TIMESTAMP behavior.
It is not tested yet.

It should be used this way:

    from kinesis.consumer import KinesisConsumer

    consumer = KinesisConsumer(stream_name='my-stream', start_at_timestamp='20190901')
    for message in consumer:
        print "Received message: {0}".format(message)

Regards,
M0dM

[codecov-io]

Codecov Report

Merging #22 into master will increase coverage by 0.58%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master      #22      +/-   ##
==========================================
+ Coverage   32.98%   33.56%   +0.58%     
==========================================
  Files           4        4              
  Lines         285      289       +4     
==========================================
+ Hits           94       97       +3     
- Misses        191      192       +1

Impacted Files	Coverage Δ
src/kinesis/consumer.py	38.46% <83.33%> (+1.05%)	⬆️

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[M0dM]

I can confirm the above code is working for me.

Regards,
M0dM