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
sure, something like

    self.endpoint = self.ENDPOINT % self.list_id

at the bottom of the constructor? I'm down for that. It's a little inconsistent with the other classes, but this class is inherently different from the others since it's a child of the RelateList class.

I almost wonder if endpoint shouldn't be a method instead of a property. You could then pull out the logic for forming the endpoint into a nice tidy place.

    @classmethod
    def endpoint(list_id, id=None)
        path = RelateList.endpoint(list_id)
        if id:
            path += '/%s' % id
        return path

Might be overkill, but would at least consolidate the url string concatenation logic into somewhere that is consistent across all classes.