Add CoinbaseOrder.render_callback
new order's status is "new", not "pending"
add function coinbase_url, check for suspicious inputs
Add order-creation methods
Restructure the tests

The response bodies are back next to the relevant tests instead of in
separate files, and most of the methods are tested with both api_key and
oauth. There are also a few more assertions now, most notably checking
query string params and request bodies.