removed refs to id builtin in favor of id_

Fixes #65 
Please see my comment from #65 --

In resource.py, `APIResource`, `@classmethod retrieve`, is it reasonable to assume no one is treating the `id` arg as a keyword argument (i.e. id=foo)? The most reasonable solution here is to rename the id arg to id_. This will break in the case where one explicitly uses id= as a keyword argument.

The only alternative I can think of which will work is to (a) change id to id_, (b) make id_ a keyword argument with a None default value, and (c) raise a TypeError if id_ is None and params.get('id') is None. This assumes an id cannot be None (which I believe is correct?).