added keys() queryset function

keys lets you restrict the fields returned by passing in names of fields you need.

parse_rest/query.py

@@ -152,6 +152,11 @@ class Queryset(object):
         q._options['skip'] = int(value)
         return q
 
+    def keys(self, *fields):
+        q = copy.deepcopy(self)
+        q._options['keys'] = ','.join(fields)
+        return q
+        
     def order_by(self, order, descending=False):
         q = copy.deepcopy(self)
         # add a minus sign before the order value if descending == True