add UrlNotResolvable exception when URL.build can't resolve url with keywords
ignore 'except' in a comment
fix rename error
able to set None as default value on filters
several of little fixes
Better filenames for save responses
Save cookies on save_responses and pave the way for more good stuff

We apparently can't override the class in the Response, even if it is
changed in the Session. Still, it will be useful to have our own class.
Save headers of response and request
Saving cookies for debug isn't very interesting

We have them in headers in a clearer way, at least for the purpose of
debugging.
Showing new cookies as they appear could be done, though.
Save request/response metadata with as much info as possible