Integrating s3walker to recursively list bucket contents starting at $prefix

Note I'm having trouble getting this to play nice in streamtools. Could you review what I've changed in listS3 and let me know where the problem is? 

I changed my s3walker to return the contents of goamz/s3's list.Contents. That is, it'll include the actual elements from list.Contents, each having a Key and LastModified attribute. I did this in the hopes of preserving the 'Since' functionality.

Finally, I didn't mean to keep my WIP toRedis block in my master branch; it's been put in a feature branch, that commit is reverted out of master as you'll see below. Thanks!