Add markdown and PyYAML to tox test requirements

Don't think there was a reason for not having `markdown` and `PyYAML` added to the test dependencies in `tox.in`. Figured it must have been forgotten a while back.

There are tests failing on Python 3 and the YAML support, but probably not many have noticed  , my guess is not many people actually using YAML with DRF and tests being skipped because of missing requirements.

I'm still not sure how to correctly fix the issue, but I think the `YAMLRenderer` should be returning a string not bytes in Python 3.

As per [this comment](https://github.com/tomchristie/django-rest-framework/pull/2076#issuecomment-63239380), I also started extracting the YAML parser and renderer as a third party package.

https://github.com/jpadilla/django-rest-framework-yaml