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?
I understand the role of IDs - Django automatically generates a PK for any models defined in `models.py`. These are the unique entities maintained by the app. These can be overwritten by the model definition to be defined by the programmer, but usually this isn't recommended (and often only done in the case of legacy databases, is this one?)

All Django Rest Framework does in this case is mimic the functionality available to the Patient model, which requires that a `uid` gets passed to it on creation (probably because this hasn't been built yet!). If the uid isn't supplied, it's the Patient model that's throwing the error, rather than the API - the API is just a layer on top the current logic.

If the `uid` is meant to be the primary key of the patient (which actually looks like it's what is trying to be achieved by the model, but the only restraints set on it by the model definition are that it is unique, rather than an actual ID), the Django model should probably indicate that. 

I'm not familiar enough with the app and its design to know whether that is the case. Implementing that shouldn't be much of an issue, but I'm hesitant to start tweaking the actual app in that way without some more knowledge of intentions and implementation guidelines.