fix: AWS SDK v3 function calls

chore: Update to PHP v7 and AWS SDK v3

chore: Update example with exp time

fix: Create iat ASAP to prevent sync issues

fix: GCM and CCM should only be available in PHP 7

feat: Include a length of iv (UInt8) into enc result

fix: Throw an error if a dataKey cannot be created

fix: Change namespace

Initial release