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. Likewise, `KeySchema` is not required.

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.

This is meant to resolve #128.