Fix the relatedTo operator in Query

Before this we were never passing in all of the data required for the
relation lookup to work.

Figured this out after looking at the 4th code example at
https://parse.com/docs/rest#queries-relational and comparing to the
request that was actually being sent.

All the tests pass now.

parse_rest/query.py

@@ -126,7 +126,7 @@ def filter(self, **kw):
             if operator is None:
                 q._where[attr] = parse_value
             elif operator == 'relatedTo':
-                q._where['$' + operator] = parse_value
+                q._where['$' + operator] = {'object': parse_value, 'key': attr}
             else:
                 if not isinstance(q._where[attr], dict):
                     q._where[attr] = {}