Add useful __repr__() methods to Model & Result

[rcoup]

Fix for #12

Uses Model.fields[0] by default, but can be overridden via Model.repr_field

eg.

>>> r = chargebee.Subscription.list({})[0]
>>> r
<chargebee.Result: customer;subscription>
>>> r.customer
<chargebee.Customer: id=1mk51RNQdn96z7hT6>
>>> r.subscription
<chargebee.Subscription: id=1mkVvvHQiQMbLBBf>
>>> r.customer.billing_address
<chargebee.BillingAddress: zip=None>