Fix documentation/cookie handling in response.set_cookie()

This updates `response.set_cookie()` with proper documentation, and refactors it so that it uses `webob.cookies.make_cookie()` instead of it's own implementation, code-reuse for the win!

However, this does break backwards compatibility because we no longer allow unicode cookie values, these were technically contra-spec in the first place, and may not have been accepted by all browsers.

We also change from `key` to `name` because cookies have names. 

This fixes #166 and #171.


Currently the API will just issue a RuntimeWarning, which hopefully means we don't break anyone, but give everyone time to update their code.

I will update this file with your suggestions for what RFC6265 allows, and I am all for doing an alpha release/beta release to make sure I didn't break anything... at least too terribly ;-)