Bunch of updates for dynamo.
Fix a bug where ConsistenRead was marshaled as a string. This caused unmarshaling errors. I'm not sure if it ended in marshaling errors on the amazon side or not.
BatchGetItem now sets its domain.
BatchWrite item now makes a copy of the headers before splitting the request. Fix a bug where headers would be set incorrectly in subsequent requests.
BatchWrite now takes in a sleep value, that if we want to space out the writes we can.
Added DescribeTable request.
Added UpdateTable request.
--- Made marshaling much easier. ---
1. If using the old way new helper methods help get items out of the map.
2. The new way will take a 'row' response and unmarshal it into a go struct. It will also marshal a go struct into a map that awsgo can marshal into json for the request body. Basic tests of this marshaling, unmarshaling in dynamo/shared_test.go.
treat time.Time objects specially. Encode them as just a string.
add a couple more tests. Just to make sure we can unmarshal when fields are missing in the marshalled source.