add REST documentation for principals module

Here's the response I get back from `/api/user/:id` for a user that has followers and is following other users. No following information is present.

```
{
  "tenant": {
    "alias": "Tenant",
    "displayName": "Tenant"
  },
  "id": "u:Tenant:xylrEJMQDp",
  "displayName": "User One",
  "visibility": "public",
  "email": "userone@not.iz",
  "locale": "en_US",
  "publicAlias": "User One",
  "picture": {
    "small": "/api/download/signed?uri=local%3Au%2FTenant%2Fxy%2Flr%2FEJ%2FMQ%2FxylrEJMQDp%2Fprofilepictures%2F1405435284144%2Fsmall.jpg&expires=1408579200000&signature=80776a041cfe1373acd53eb30ef7a73992a063f7",
    "medium": "/api/download/signed?uri=local%3Au%2FTenant%2Fxy%2Flr%2FEJ%2FMQ%2FxylrEJMQDp%2Fprofilepictures%2F1405435284144%2Fmedium.jpg&expires=1408579200000&signature=fe9e8a5df074a66d0330a095e8ac3cd1e95281f1",
    "large": "/api/download/signed?uri=local%3Au%2FTenant%2Fxy%2Flr%2FEJ%2FMQ%2FxylrEJMQDp%2Fprofilepictures%2F1403542881749%2Flarge.jpg&expires=1408579200000&signature=50d2f014102ae64242c072765e032cfaddc362e4"
  },
  "profilePath": "/user/Tenant/xylrEJMQDp",
  "resourceType": "user",
  "notificationsUnread": 0,
  "notificationsLastRead": 1407418942947,
  "acceptedTC": 1400523404749,
  "lastModified": 1407418942948,
  "emailPreference": "daily",
  "isGlobalAdmin": false,
  "isTenantAdmin": true
}
```