Validate MD5 checksums in SQS operations.

This feature can be disabled by passing `computeChecksums: false`
to the SQS service object constructor.

Affected operations are:

* sendMessage
* sendMessageBatch
* receiveMessage

If any messages sent or received in these operations fail the MD5
check, an error will be raised for the affected message.
Add messageIds to error to track corrupt messages
Refactor tests
More test refactoring
Append message ID to error.messageIds for sendMessage
Merge pull request #124 from aws/sqs-checksums

Validate MD5 checksums in SQS operations.