Adding new feature

Now it's possible to add a description to an alias. That will be passed to Grunt.

For example:

````
default:
  - 'test'
anotherTask:
  description: 'This is an awesome task'
  tasks:
    - 'foo'
    - 'bar'
````

Will then be passed to grunt so running `grunt --help` will show the description.

> Note that the syntax is not exactly the same as proposed in #39 since `registerTask`, when receiving three parameters, receive a function and not a task list according to [their documentation](http://gruntjs.com/api/grunt.task#grunt.task.registertask). 

Fixes #39