CreationDateTime must be an optional field in TableDescription

See the DynamoDB documentation which specifies this field as not
required:
http://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_TableDescription.html#DDB-Type-TableDescription-CreationDateTime

Previously, if a DeleteTable request responded with a table description
which lacked CreationDateTime (which can happen), an exception would be
thrown in parsing the response.
KeySchema is also optional
Merge pull request #129 from alephcloud/fix/optional-CreationDateTime

CreationDateTime must be an optional field in TableDescription