Implemented saving pixmap with 1 or 3 channels

Only place that needed change was the convert implementation, other things look ready. I tested with "synthetic" pixmaps like this one:

```js
var pixels = new Buffer(400);
pixels.fill(0xcc);

var pixmap = { format: 2,
  width: 20,
  height: 20,
  rowBytes: 20,
  colorMode: 1,
  channelCount: 1,
  bitsPerChannel: 8,
  pixels: pixels,
  bytesPerPixel: 1,
  bounds: { top: 0, left: 0, bottom: 20, right: 20 }
};

generator.savePixmap(pixmap, 'pixmap.png', { format: 'gif', ppi: 72 });
```

This should make it easier to implement mask support as discussed in https://github.com/adobe-photoshop/generator-core/issues/161#issuecomment-56403631

I tested output to png, gif and jpg and all seems working perfectly.