RecursiveRelatedField: First draft of implementation, tests and docs

An attempt of implementing a related field that handles serialization of recursive `ForeignKey` or `OneToOne` fields, partly based on [this thread](http://stackoverflow.com/a/13377226/1326146). Quick changes overview:

* Updated `api-docs/relations.md`.
* Updated `relations.py`.
* Added `tests/relations_recursive.py`.