Add abilities helper that serializes cancan permissions

As discussed in #1 this PR adds a new helper method to ActiveModel::Serializer objects.  I didn't add it to the `meta` because permissions can be dependent on the actual object being serialized.  I did, however, make it so only one attribute, called `abilities`, is added to the serialized hash.  This should make it easy for various JS frameworks to have a single point of entry for creating an interface to the ability information.  The README changes show examples.