Fix CondOp JSON rendering for nullary conditional operators

AttributeValueList must only be rendered when it is to contain at least
one element. Rendering an empty AttributeValueList will result in a 500
error; this occurs when using a nullary conditional operator such as
IsNull or NotNull.
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
tests/Utils.hs: add a version of retryT that returns the number of attempts
tests/Utils: change context of tryT to MonadBaseControl IO m
Test utils for DynamoDb
Merge pull request #127 from alephcloud/fix/conditional-writes

Fix CondOp JSON rendering for nullary conditional operators
Minimal dynamoDb test suite

Currently only basic table creation, description, and deletion is covered.
regenerate aws.nix
Merge pull request #131 from alephcloud/dynamodb-test-suite

Dynamodb test suite
add time prefix to all test resources
add more debug output to help find dynamodb issue
tests/Utils.hs: add function 'mustFail'
tests/Sqs/Main.hs: add function SqsT to run requests with a given manager and base configuration
tests/DynamoDb/Utils.hs: add function dyT to run requests with a given manager and base configuration
tests/DynamoDb: add test case for reuse of TCP connections
tests/Sqs: add test case for reuse of TCP connections
DynamoDb: trigger reuse of TCP connection by forcing endOfInput in ResponseConsumer
Merge pull request #129 from alephcloud/fix/optional-CreationDateTime

CreationDateTime must be an optional field in TableDescription
fix bug in parsing of DeleteTable responses