Links as object for JSON representation

First of all, great work with this library!

According to the examples in

http://stateless.co/hal_specification.html

the _links section of a HAL-JSON should be represented by a JSON-object rather than a collection. Furthermore links with the same 'rel' should be an entry in the object with the 'rel' as key and a collection with all the hrefs as values. I don't know if this is a rule of any kind or just a matter of taste, but I like this representation better than the collection one.

These commits implements the JSON-representation in this way.