Add `MultiSlugRelatedField`

Adds a new subclass of `serializers.RelatedField` which allows for the
representaion of a target using multiple fields.

I took a stab at this while looking at [Issue #1666](https://github.com/tomchristie/django-rest-framework/issues/1666).  

- Very interested in feedback on whether I went the right direction on this.
- I modeled a lot of what I did off of `SlugRelatedField`
- I'm not in love with the name `MultiSlugRelatedField` so any suggestions on a better name are appreciated.