Added support for an options hash for links

Fixes #16 by offering the ability to declare additional options when
fetching a related model. Instead of including the model after the rel
name, include a hash containing the model and supplementary options.

For example:

links: {
  'some-rel': Backbone.Model,
  'another-rel': {
    model: Backbone.Model,
    options: {
      follow: [ 'another-rel' ]
    }
  }
}
																	                        }
Merge branch 'feature-allow-options-for-links'
Merge branch 'master' of github.com:linn/backbone.hypermedia