Load images via URL in Node

This small PR allows to load image by passing its url to the `Caman` constructor:

```javascript
Caman("http://path/to/image.jpg", function () {
  ...
});
```

This behaviour is already possible in the browser, so I think I'd be nice to have the same functionality on the server. 
Uses only core `http` module.

I wanted to update the documentation as well, but wasnt sure where are the sources.