Add MD5 validation for Amazon SQS.

When available, validates SQS operations against MD5 checksums,
raising errors when there are mismatches.
Configuration toggle for SQS MD5 check.

Adds :sqs_verify_checksums to the AWS config. True by default,
setting this value to false will skip the validation of MD5
checksums for SQS operations.
Remove SQS checksum verification from global config.

Accidentally left the old location of :sqs_validate_checksums
out of the pull request. Removing the old version, tests still
pass.
Merge pull request #332 from aws/sqs-checksum

MD5 Checksum Validation for Amazon SQS