MAX_AGE_AV regexp: account for Max-Age=0

Some of the web frameworks (e.g. Plone) use the following trick to clear cookies after logout:
Set-Cookie: auth=deleted; Max-Age=0 
cookies.py currently chokes in those cases because the RFC forbids that (the leading digit must be non-zero).
Merge pull request #6 from ngo/master

MAX_AGE_AV regexp: account for Max-Age=0