Changed the inheritance model to remove types

The inheritance model has changed, we no longer need to store an array of
`types` with the model we can just use the classname in `_cls`.

See the upgrade docs for information on how to upgrade
MongoEngine/mongoengine#148

mongoengine/queryset/field_list.py

@@ -0,0 +1,51 @@
+
+__all__ = ('QueryFieldList',)
+
+
+class QueryFieldList(object):
+    """Object that handles combinations of .only() and .exclude() calls"""
+    ONLY = 1
+    EXCLUDE = 0
+
+    def __init__(self, fields=[], value=ONLY, always_include=[]):
+        self.value = value
+        self.fields = set(fields)
+        self.always_include = set(always_include)
+        self._id = None
+
+    def __add__(self, f):
+        if not self.fields:
+            self.fields = f.fields
+            self.value = f.value
+        elif self.value is self.ONLY and f.value is self.ONLY:
+            self.fields = self.fields.intersection(f.fields)
+        elif self.value is self.EXCLUDE and f.value is self.EXCLUDE:
+            self.fields = self.fields.union(f.fields)
+        elif self.value is self.ONLY and f.value is self.EXCLUDE:
+            self.fields -= f.fields
+        elif self.value is self.EXCLUDE and f.value is self.ONLY:
+            self.value = self.ONLY
+            self.fields = f.fields - self.fields
+
+        if '_id' in f.fields:
+            self._id = f.value
+
+        if self.always_include:
+            if self.value is self.ONLY and self.fields:
+                self.fields = self.fields.union(self.always_include)
+            else:
+                self.fields -= self.always_include
+        return self
+
+    def __nonzero__(self):
+        return bool(self.fields)
+
+    def as_dict(self):
+        field_list = dict((field, self.value) for field in self.fields)
+        if self._id is not None:
+            field_list['_id'] = self._id
+        return field_list
+
+    def reset(self):
+        self.fields = set([])
+        self.value = self.ONLY