Added multiple source images and manual strategy

I needed a way to stitch images together so I added the ability to download multiple source images and a manual strategy to give me flexibility.

This required modification of the client. So this is a breaking change. Both client and server would have to be updated.

Example use:
```javascript
var resources = [
  'images/image1.png',
  'images/image2.png',
  'images/image3.png'
];

var taskDescriptions = [{
  suffix: 'stitch', 
  exec: "%(command)s -border 0 -tile 3x1 -geometry 160x106 "%(sources[0])s" "%(sources[1])s" "%(sources[2])s" -quality 90 %(output)s",
  command: 'montage',
  format: 'jpg',
  strategy: 'manual'
}];

thumbdClient.task(resources, 'images/save-file-prefix', taskDescriptions);
```