Multiple Model Enhancements (#65)

* models: Host now has a source attribute.

The new source attribute is an optional field on the Host model. In the
future, when the field is not empty, the value will be used when
deciding which StoreHandler to utilize when populating data for the
instance.

For more information see
http://commissaire.readthedocs.io/en/latest/cpd/61.html#model-changes

Closes #64.

* models: Added string representation method.

* models: Added missing doc for primary_key property.

* fixup! models: Added string representation method.

src/commissaire/models/__init__.py

@@ -98,10 +98,25 @@ def new(cls, **kwargs):
         instance.__init__(**init_args)
         return instance
 
+    def __str__(self):  # pragma: no cover
+        """
+        Returns a string representation of the instance.
+
+        :returns: String representation of this instance.
+        :rtype: str
+        """
+        readable = '<{}'.format(self.__class__.__name__)
+        for k, v in self.to_dict_safe().items():
+            readable += ' {}={},'.format(k, v or '<Empty>')
+        return readable + '...>'
+
     @property
     def primary_key(self):  # pragma: no cover
         """
         Shortcut property to get the value of the primary key.
+
+        :returns: The primary key value.
+        :rtype: mixed
         """
         return getattr(self, self._primary_key)
 
@@ -481,11 +496,12 @@ class Host(Model):
         'last_check': {'type': str},
         'ssh_priv_key': {'type': str},
         'remote_user': {'type': str},
+        'source': {'type': str},
     }
     _attribute_defaults = {
         'address': '', 'status': '', 'os': '', 'cpus': 0,
         'memory': 0, 'space': 0, 'last_check': '', 'ssh_priv_key': '',
-        'remote_user': 'root'}
+        'remote_user': 'root', 'source': ''}
     _hidden_attributes = ('ssh_priv_key', 'remote_user')
     _primary_key = 'address'