Adding URL openers that support automated cookie handling

See urllib3/contrib/cookies.

Basically, I created a class that can take strings from the Set-Cookie header from a request and put them in a jar. The cookies can also be extracted and converted to a Cookie header string.

Then, I subclassed the ConnectionPools and overrode their urlopen methods to send a Cookie header using cookies from my class, and feed the Set-Cookie headers from the response into my class.