S3: support multi-part uploads

[matthewkmayer]

Related to #18. See http://docs.aws.amazon.com/AmazonS3/latest/dev/mpuoverview.html and http://docs.aws.amazon.com/AmazonS3/latest/API/mpUploadUploadPart.html .

[matthewkmayer]

We can follow the high level Java example and just accept a BufStream and report back when it's done. Rusoto will handle keeping track of parts and putting them all together. See http://docs.aws.amazon.com/AmazonS3/latest/dev/usingHLmpuJava.html .

Perhaps make a helper function in S3Helper that can take a file as input. This would call the function that accepts a BufReader as input.

[matthewkmayer]

Since this can turn complicated when adding threads to do parallel uploads, this ticket is for single threaded multi-part upload. Should handle resuming of uploads by checking what's already uploaded.