Updated docstring for ``orm.Entity.Meta``.

The docstring was outdated so I've updated it.
Added ``Ping`` and ``Status`` entities.

I have added the following new Foreman entities:

* ``Ping``

```python
{u'services': {u'candlepin': {u'duration_ms': u'47', u'status': u'ok'},
  u'candlepin_auth': {u'duration_ms': u'41', u'status': u'ok'},
  u'elasticsearch': {u'duration_ms': u'30', u'status': u'ok'},
  u'katello_jobs': {u'duration_ms': u'45', u'status': u'ok'},
  u'pulp': {u'duration_ms': u'16', u'status': u'ok'},
  u'pulp_auth': {u'duration_ms': u'34', u'status': u'ok'}},
 u'status': u'ok'}
```

* ``Status``

```python
{u'release': u'Katello',
 u'timeUTC': u'2014-07-25T19:43:21Z',
 u'version': u'1.5.0-27.el6sat'}
```

Since these new entites only expose a ``GET`` method, there's no need to
define their fields.
One more tweak to ``orm.Entity.Meta`` docstring.
Merge pull request #1102 from omaciel/new-entities

New entities and docstring change