Added ability to type hint a task

Added ability to type hint a Grunt task so that syntax like:

    <?php

    use League\Grunt\Grunt;

    return [
        'hosts' => [
            'production' => 'foo.com'
        ],
        'tasks' => [
            'deploy' => function(Grunt $g) {
                $g->run('ls');
            }
        ],
    ];

Can be used. This gives the ability to autocomplete methods from an IDE, as well as aid in using debuggers.
Merge pull request #4 from mrkrstphr/allow-type-hinting

Added ability to type hint a task