Fixes for RelateListItem and POST requests

Thanks for creating a great starting point for RelateIQ integration! This pull request is probably 90% fixes for things that didn't work at all, and 10% cleanup.

* Fix JSON data encoding for POST and PUT requests (don't think this ever worked)
* Fixing 'set_field' method for RelateListItem (would always raise an Exception)
* Fixing RelateListItem.save() for new RelateListItem instances (don't think this ever worked)
* Fixing RelateListItem.get_by_id() (don't think this ever worked)
* Fixing 'title' in RelateList.update_from_dict (would not get title)
* Removed unused 'LIST_ID' property of RelateObject
A method to determine the endpoint would be ideal, but we'd be overcomplicating a beta api wrapper :)

I'll make a note for the next time I'm refactoring to revisit this.