Initial implementation of poster

This sends a response to the requested callback URL once a job is done.
Unexport internal field

We don't want the S3 bucket visible in the JSON response, since it's
redundant with BucketName and more importantly its marshalled
representation includes our amazon credentials!
Tag struct to get nicer json representation
Push the job down to a second level object

The details should be there for inspection and correlation but are not
the primary facts the remote client cares about.