title: How to validate nested objects in cerberus?
body: <p>I have the following schema:</p>
  
  <pre><code>schema = {
      'person': {
          'name': {'type': 'string', 'required': True, 'minlength': 2},
          'gender': {'type': 'integer', 'required': True},
          'weight': {'type': 'integer', 'required': True},
          'age': {'type': 'integer', 'required': True, 'min': 5}
      },
      'parameters': {
          'ps': {'type': 'float', 'required': True},
          'pp': {'type': 'float', 'required': True},
          'ls': {'type': 'float', 'required': True},
          'lp': {'type': 'float', 'required': True},
          'ab': {'type': 'float', 'required': True}
      },
      'indicators': {
          'atmospheric_pressure': {'type': 'integer', 'required': True},
          'puls': {'type': 'integer', 'required': True},
          'respiration_rate': {'type': 'integer', 'required': True}
      }
  }
  </code></pre>
  
  <p>When I run validation:</p>
  
  <pre><code>v = Validator(schema)
  </code></pre>
  
  <p>I get an error:</p>
  
  <pre><code>Traceback (most recent call last):
    File "index.py", line 53, in &lt;module&gt;
      v = Validator(schema)
    File "C:\Users\Oleg\AppData\Local\Programs\Python\Python36\lib\site-packages\cerberus\validator.py", line 143, in __init__
      self.schema = kwargs.get('schema', None)
    File "C:\Users\Oleg\AppData\Local\Programs\Python\Python36\lib\site-packages\cerberus\validator.py", line 472, in schema
      self._schema = DefinitionSchema(self, schema)
    File "C:\Users\Oleg\AppData\Local\Programs\Python\Python36\lib\site-packages\cerberus\schema.py", line 56, in __init__
      self.validate(schema)
    File "C:\Users\Oleg\AppData\Local\Programs\Python\Python36\lib\site-packages\cerberus\schema.py", line 181, in validate
      self._validate(schema)
    File "C:\Users\Oleg\AppData\Local\Programs\Python\Python36\lib\site-packages\cerberus\schema.py", line 203, in _validate
      raise SchemaError(self.schema_validator.errors)
  cerberus.schema.SchemaError: {'indicators': [{'atmospheric_pressure': ['unknown rule'], 'puls': ['unknown rule'], 'respiration_rate': ['unknown rule']}], 'parameters': [{'ab': ['unknown rule'], 'lp': ['unknown rule'], 'ls': ['unknown rule'], 'pp': ['unknown rule'], 'ps': ['unknown rule']}], 'person': [{'age': ['unknown rule'], 'gender': ['unknown rule'], 'name': ['unknown rule'], 'weight': ['unknown rule']}]}
  </code></pre>
  
  <p>Also i tried to specify type:</p>
  
  <pre><code>schema = {
      'person': {
          'type': 'object',
          'schema': {
              'name': {'type': 'string', 'required': True, 'minlength': 2},
              'gender': {'type': 'integer', 'required': True},
              'weight': {'type': 'integer', 'required': True},
              'age': {'type': 'integer', 'required': True, 'min': 5}
          }
      },
  </code></pre>