Add "external_datasets" field to Revision

[evantayloryates]

Resolves #45

If you look at the code for Revisions.new:

@staticmethod
def new(auth, metadata):
    path = 'https://{domain}/api/publishing/v1/revision'.format(
        domain = auth.domain,
    )

    response = post(
        path,
        auth = auth,
        data = json.dumps({
            'action': {
                'type': 'update'
            },
            'metadata': metadata
        })
    )
    return Revision(auth, response)

The revision's JSON data has two top-level attributes, action and metadata. However, if you look at Socrata's Publishing api (which socrata-py wraps around), you'll see that in order to add an external URL, you need a third top-level attribute, href (pictured below).

I've added a small amount of code to allow users to specify external datasets when creating new Revisions