Fixes #416 - accepts hash as a list of arguments.

Fixes #416 - accepts hash as a list of arguments.

Such a hash is now accepted as the first argument to `minify` method:

```javascript
{
  'path/to/file/one': {
    styles: 'contents of file one'
  },
  'path/to/file/two': {
    styles: 'contents of file two'
  }
}
```

This is also prepares us to implement #419 - merging input source maps.