don't use relative path to spin.js dep

Problem:
If you're already using spin.js in your app, it would get loaded twice. Once from the path defined in your app's requirejs config and once from the relative path defined by Ladda.

Solution: Don't use relative paths in a library like Ladda. (It also allows consumers of Ladda to use a package manager like bower to install spin.js and potentially Ladda)

Downside: For apps that don't already use spin.js the author would have to set up a path mapping for requirejs like this:
```
paths: {
  "spin": "path/to/spin"
}
```

This should be added to the docs.

Does that make sense?