Add fetch_link to make it much easier to follow linked URLs within an API

[bdowling]

This is an attempt at answering the question I had in #352

By simply adding x-operationId: [linkedOperation] in your spec to URLs that are returned from the API that are internal links, and adding add_fetch_link: True to your config, those URLs will be returned in the model as FetchLink objects that can be called to retrieve the named `[linkedOperation]' on the given URL (e.g. obtain the http_request on that url and operationId, as if a CallableOperation had been called).

I think this is fairly straightforward. The only item I question is how I added the backref to swagger_spec._client, mainly so I could access my new SwaggerClient._get_operation from the context of http_future where it seems I only had access to the operation and spec. If there was some other way around this, please let me know. Alternatively _get_operation could be added to bravado-core instead I suppose. I just felt it was similar in vein to _get_resource that is in this module.

Without the new config option, there is no changed default behaviors.

I suppose I need to add some tests for this to be accepted, but figured I'd throw it out here to get some feedback.

[bdowling]

Example usage:

while 'next' in data:
    url = data['next']
    if url is None:
        break
    print("URI: ", repr(url))
    nextr = url()
    data,res = nextr.result()

  MultipleInstrumentData:
    type: object
    properties:
      count:
        type: integer
        format: int32
      next:
        type: string
        format: url
        x-operationId: 'instruments'
        x-nullable: true
      previous:
        type: string
        format: url
        x-operationId: 'instruments'
        x-nullable: true
      results:
        type: array
 ....

[coveralls]

Coverage decreased (-3.1%) to 95.462% when pulling b577eb0 on bdowling:add_fetch_link into 510c7ce on Yelp:master.

[coveralls]

Coverage decreased (-3.1%) to 95.462% when pulling b577eb0 on bdowling:add_fetch_link into 510c7ce on Yelp:master.

[coveralls]

Coverage decreased (-3.1%) to 95.462% when pulling b577eb0 on bdowling:add_fetch_link into 510c7ce on Yelp:master.

[coveralls]

Coverage decreased (-3.1%) to 95.462% when pulling b577eb0 on bdowling:add_fetch_link into 510c7ce on Yelp:master.