Add `MultiSlugRelatedField`

Adds `MultiSlugRelatedField` a new subclass of `serializers.RelatedField` which allows for the
representation 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` and it's test suite.
- I added 3 new models to the test models to `rest_framework.tests.models`.  I didn't see any there that had a `unique_together`.  Let me know if I should go a different direction with this if adding these models is not desirable.
- I'm not in love with the name `MultiSlugRelatedField` so any suggestions on a better name are welcome.

Here is a cute animal picture for your troubles.

![baby-giraffe-2-12641](https://cloud.githubusercontent.com/assets/824194/3560763/55e05b90-098a-11e4-8369-7969ada92679.jpg)