Http callback when job is complete

I implemented a http callback notification for when a job is complete. This is based on my previous pull request #34. Sorry about that. I know it makes it difficult when people create cascading pull requests. thumbnail.task has been renamed to thumbnail.job. I thought that made more sense.

example usage: 
```javascript
var job = {
  resources: ["images/image1.png"],
  prefix: "image/destination-prefix",
  descriptions: [{
    ...
  }],
  notify: "http://url.to.post/when/job/is/complete"
}

thumbdClient.job(job);
```

The endpoint would receive a post containing the job with a list of all output files. 

My use case for this is to then purge the modified files from cache.