move definitions to watch task from jshint

To define files, 'watch' task is a better location than 'jshint'.

See a example about handling 'watch' event:

```
grunt.event.on('watch', function(action, files, target){
  ...

  // change jshint config
  grunt.config(['jshint', target], {src: files})

  ...
}
```

Once watch event fired, jshint's configuration is changed. If watch
task refer jshint's configuration, watching files also is changed.