Add support for selecting specific fields of Etsy resources, and requesting associations

[alekstorm]

Association is a bit of a long name, but there isn't an immediately apparent way to shorten it. Making Bounds an inner class of Association doesn't help matters, but it doesn't make sense when top-level.

The serialization method uses the ''.join(list) trick to speed up serialization (ostensibly) by an order of magnitude, but this may be a premature optimization.