Adding debug mode and some more documentation

A basic version of this (returns url called, data passed to the api call, and headers) should now be available in pypi version 3.1.1.

You can access this by passing a truthy value to the kwarg debug to either the constructor of a client or by passing that to the call you're making.

example:

deals_client = DealsClient(api_key=API_KEY, debug=True)
deals_client.get_all(propertiesWithHistory="Amount")

# or this works too
deals_client = DealsClient(api_key=API_KEY)
deals_client.get_all(propertiesWithHistory="Amount", debug=True)

Thanks @guysoft for the suggestion and some documentation for the readme!