Update datatypes to check for circular pointer references. Change circul...

Fixed Pointer Object to check for circular references. ParseResource now passes class name (the Parse class name once resolved) to the ParseType.convert_from_parse method. 

In turn, when a pointer is come across, the "parent class name" is now passed to the Pointer object which checks for circular references. If a pointer object comes back and has an attribute that is a pointer to the parent class, then we replace this element with a string id. 

This may not be the best solution for long term, but it works fine for my application where I'm using many circular references. Let me know any feedback you may have. 

https://github.com/dgrtwo/ParsePy/issues/28