allow json retrieval

Depends on task 1

By doing

resource = Restfulie.at('entryuri').get

Where the resource is an application/json, text/json as, i.e:

{"item":{"name":"product", "price":2}}

resource.item.name # product
resource.item.price # 2

Try using the most important xml unmarshaller in python, in a way it can be changed later:

The get method will retrieve the result, check that "raw" was not invoked, look the Content-type header, and according to its result, using a registering hash, map to a XmlUnmarshaller