OAuth2 API

Adds to the `authentication` API in order to support zf-oauth2.

oauth2 resources are represented by:

```javascript
  {
      "dsn": "PDO DSN of database containing OAuth2 schema",
      "username": "Username associated with DSN",
      "password": "Password associated with DSN",
      "route_match": "Literal route to match indicating where OAuth2 login/authorization exists"
  }
```

The `AuthenticationEntity` is now aware of OAuth2 configuration, and the `AuthenticationModel` is as well.