Add a relationships() method to url builder

[ghunti]

Hi

According to JSON API spec, you can update a relationship by using the "relationships" link ex post/1/relationships/comments.
Currently Devour only allows this through the resource link post/1 by adding the respective relationship object in the data sent to the server.
If we add a method similar in behavior to all and one to the url builder we could allow relationships links to work.
Ex

relationships () {
    this.builderStack.push({path: 'relationships'})
    return this
}

After this we could do something like jsonApi.one('post', 1).relationships().all('comments').post([{id: comment1Id}, {id: comment2Id}])

The "relationships" url could be configurable to allow more flexibility.
What do you think?

[Emerson]

This seems legit and I'd be happy to have it in our library.

[GregPeden]

This was added by this PR
#99

I suggest this issue may be closed.