Change grunt folder to tasks

To make use of task loading in NPM:

```
grunt.loadNpmTasks('intern');
```

The folder name must be tasks, according to the grunt source code.

```
  // Process task plugins.
  var tasksdir = path.join(root, name, 'tasks');
  if (grunt.file.exists(tasksdir)) {
    loadTasks(tasksdir);
  } else {
    grunt.log.error('Local Npm module "' + name + '" not found. Is it installed?');
  }
```