Fixed NSDictionary+Subscript to use `objectForKey:`

NSDictionary+Subscript was using the KVC method `valueForKey:` for its `objectForKeyedSubscript:` method instead of the NSDictionary method `objectForKey:`.  Switched to use `objectForKey:`.

Resolves paneidos/objective-c-subscript#5
Merge pull request #6 from capnslipp/master

Fixed NSDictionary+Subscript to use `objectForKey:`