Mega feature branch

Hi Ryan,

Apart from my gutting Virtus (I see you fixed it, sorry about that), this should be compatible.

Today, I was spiking the eBay Finding API. A few brief comments:

- I'm wondering if there's any value in relaxing the response data format and allowing JSON. Parsing into JSON or XML based on the Content-Type header should be quite simple.
- At least the Finding API allows passing the current default parameters as HEADER key values. Not that anyone's watching, but this might be a slightly cleaner and more secure way to at least keep the API token from prying eyes.
- At least the Finding API works without specifying the VERSION. It defaults to the latest.

There were probably bunch of other things, but I have to rush home now.

:metal: