Add conditional writes

[willglynn]

Amazon S3 now supports conditional writes (blog post). Docs indicate that s3:PutObject and s3:CompleteMultipartUpload now support the request header If-None-Match: *, in which case S3 will:

1. return a 200 if it created the object did not exist and was created as requested,
2. return a 412 Precondition Failed if the object already exists, or
3. return a 409 Conflict if the object is deleted before creation completes

This upstream change updates #218.