Make a POST request to this resource with `username` and `password` fields to
obtain an authentication token to use for subsequent requests.

Example JSON to POST (content type is `application/json`):

    {"username": "user", "password": "my pass"}

Example form data to post (content type is `application/x-www-form-urlencoded`):

    username=user&password=my%20pass

If the username and password provided are valid, the response will contain a
`token` field with the authentication token to use and an `expires` field with
the timestamp when the token will expire:

    {
        "token": "8f17825cf08a7efea124f2638f3896f6637f8745",
        "expires": "2013-09-05T21:46:35.729Z"
    }

Otherwise, the response will indicate the error that occurred and return a 4xx
status code.

For subsequent requests, pass the token via the HTTP `Authorization` request
header:

    Authorization: Token 8f17825cf08a7efea124f2638f3896f6637f8745

Each request that uses the token for authentication will refresh its expiration
timestamp and keep it from expiring.  A token only expires when it is not used
for the configured timeout interval (default 1800 seconds).
