Allow relationships as array

[qrosmeli]

Hello.

First, I want to thank you for this code, it save us some time in our migration to JSON API.

I added the option to leave relationships as an array (just like normalizr), with a flag as a second argument (default to false).

[sl-ffx]

@qrosmeli I'm just missing one thing in your PR:

Quote from Readme

Note that, unlike normalizr, this library will always point to associations using the format { type: 'users', id: 4 }.

I think this should be an array of objects with type and id as attributes.

And hopefully @stevenpetryk can merge this PR asap.
This feature would be great to have.