Avoid to open all documents from cursors in an if stmt

Using a cursos in an if statement:

cursor = Collection.objects

	if cursor:
		(...)

Will open all documents, because there are not an __nonzero__ method.
This change check only one document (if present) and returns True or False.

mongoengine/queryset/base.py

@@ -154,6 +154,15 @@ class BaseQuerySet(object):
     def __iter__(self):
         raise NotImplementedError
 
+    def __nonzero__(self):
+        """ Avoid to open all records in an if stmt """
+
+        for value in self:
+            self.rewind()
+            return True
+
+        return False        
+
     # Core functions
 
     def all(self):