Add RSpec and Rake to development dependencies

Let's get some spec coverage in here.

Signed-off-by: Stephen Celis <stephen@stephencelis.com>
Add basic Rakefile

To run specs.

Signed-off-by: Stephen Celis <stephen@stephencelis.com>
Add basic spec coverage

We were missing some requires (which wouldn't be obvious in most
applications, which already require ActiveSupport).

This also removes the _links node unless at least one link is specified.

Signed-off-by: Stephen Celis <stephen@stephencelis.com>
Require link handler

Links require a block right now, so let's blow up early.

We may want to explicitly raise unless block_given?, but this is
shorter.

Signed-off-by: Stephen Celis <stephen@stephencelis.com>