Be able to call resource.links with argument when no links are present

This fixes an issue with resources without links. The issue can be reproduced like this:
* Given an unparsed resource without a _links property,
* when it is parsed
* and you call the links method with an argument (like `resource.links('whatever')`) to check if a specific link is there
* you get the following error: `TypeError: Object #<Links> has no method 'get'`
* Expectation: No error, instead `resource.links('whatever')` should return null or undefined.

The problem is that the Links constructor returns on empty input before the get method is even defined and that the resource object (in its `links` method) relies on the get method to be always there. This PR changes the order in the Links constructor so that the Links object always has a get method.