Add blade directives

Hello,

I have many views which conditionally include sub views. Some of those sub views require their own assets. Instead of duplicating the logic in both the views and controllers, I put the Asset calls in the views themselves. Because of this, I found myself littering my blade files with snippets of `<?php Assets::add('somefile'); ?>` and figured it would look nicer if I could include assets with a Blade Directive. So I added them!

Let me know if this is something you would be interested in having as part of the package.

The reason I put it in the Service Provider is so the directives are only registered if it is actually included in a Laravel Application. If this is not the best place to put them, or you would not have created a separate method let me know. I am up to change it around.

Thank you for the awesome package,
Kevin Gravier