Dynamodb error handling

Hi,

These two commits update the error handling for the dynamodb support.

The first commit uses the global logger (log.Printf) instead of stdout (fmt.Printf). This is useful in libraries since in a production app the global logger may be set up to go elsewhere (e.g. we use the 'timber' logger).

The second commit checks for failed http responses from dynamodb and propogates them as errors, copying the approach taken by goamz/s3.

Hope these changes meet your approval, please let me know if you'd like them reworked and/or extended.

regards,

jb