Basic and extendable API for patients

A simple API using django-rest-framework to full-fill #3.

# What's changed:
* Add token authentication
* Add a patient serializer
* Add a patient API viewset
* Add URL endpoints `api` and and `api/patients`
* Add search on patients ex: `api/patients?search=Jane`
* Add filter on patients ex: `api/patients?alive=True`
* Some basic documentation in README.md in `api/`
* Some basic tests

# What's next
* Wait on feedback, add more filters, more search parameters?
The problem with IDs is that there is a WHO standard on how these are created. We are currently implementing a ID service that will generate such IDs. They are quite complex and have loads of rules that should make it easy for them to be read out over the phone. They also have a checksum. So we need to use this id :( Even if the pk would be a far easier solution IMHO