Add Associations

Please see tests in pull request for examples of how to use associations & relations.

Due to restraints the parse API, we must make an additional query when saving and updating objects that have relations, this is so we can fetch the included objects from the server

In this implementaiton, this extra query will not be added unless the model has relations (has_many in particular).

This implementation supports both relation column types, and array columns that have pointers in it. In both cases child objects will be pulled from parse and filled as relations.

This implementation will also automatically include child objects when querying.

I'd like to thank @kkelani for his hard work on this plugin back in 2012, This pull request is an updated version for the latest version of parse_resource. And ready to be merged. All current Parse_resource tests pass as well as these new relation tests.