Add the concept of token authentication to the API

Install the `rest_framework.authtoken` app which allows you to add 
authentication tokens for particular users. This will allow us to make eg a
`surge-model-collector` user with their own token.

We've currently got only read-only endpoints so this shouldn't have any 
effect, but it is required for implementing the writeable surge/measurement
endpoints.

https://www.pivotaltracker.com/story/show/78037278