Resource identifier objects, override top level links.

[RazerM]

Still needs documentation, do not merge.

This PR adds a resource_identifier keyword argument to Schema to dump resource identifier objects, as you would at a relationships endpoint, e.g. /users/<user_id>/relationships/articles.

In cases like this, both self and related URLs are different from the ArticleSchema we'd use to dump the data.

schema = ArticleSchema(
    many=True,
    resource_identifier=True,
    self_url='/users/1/relationships/articles',
    related_url='/users/1/articles',
)

The Flask extension uses views:

from flask import request

@app.route('/users/<int:user_id>/relationships/articles')
def user_articles_relationship(user_id):
    schema = ArticleSchema(
        many=True,
        resource_identifier=True,
        self_view=request.endpoint,
        self_view_kwargs=request.view_args,
        related_view='user_articles',
        related_view_kwargs={'user_id': user_id},
    )

The logic for this was put into get_top_level_links, like was done when the self_url and self_url_many options were added.

[RazerM]

Would this be merged if I added documentation, or does this not fit your API?

[RazerM]

bump?