Session management: login

[chrisabruce]

Authenticates the user.

Authenticates the user, and issues an access token they can use to authorize themself in subsequent requests.

If the client does not supply a device_id, the server must auto-generate one.

The returned access token must be associated with the device_id supplied by the client or generated by the server. The server may invalidate any access token previously associated with that device. See Relationship between access tokens and devices_.

[dr-bonez]

Default expiration?