repr() methods #12

Currently in an interactive shell/debugger/traceback you don't get a very good representation of the model instances:

eg. <chargebee.models.customer.Customer at 0x7ff7f9fb0e48>

Would be helpful to make it reference the Chargebee IDs by implementing Model.__repr__()

eg. <Customer: ZEw32jZRMQ0bB21A9t>

Implementation idea:

1. Assume Model.fields[0] is the key to use for representing the instance.
2. For most models this will be id (or entity_id/cn_id/txn_id/etc), or could be by simply re-ordering the fields list
3. For things like addresses without a natural key it could be a zip code? Or alternatively subclass those from an AddressModelMixin class which implements __repr__() by merging a few fields together (eg. return " ".join([self.city or '', self.zip or '', self.country or '']))