Improve apiRoute method, including a new only() method

[cbj4074]

Sometimes, it's more useful to specify which API methods we do want, vs. those we don't.

For example this is what's required currently to have only a show method:

$this->apiRoute('featured', 'FeaturedController')
    ->except(['index', 'store', 'update', 'destroy'])
    ->addDefaultRoutes();

This would be much cleaner:

$this->apiRoute('featured', 'FeaturedController')
    ->only(['show'])
    ->addDefaultRoutes();

Also, make any other improvements/adjustments, as necessary, based on experiences thus far.