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.


Can you say a few words about what RFC6265 allows here so folks don't need to look up the spec, and say under what circumstances a warning will be issued?