urllib-baseb implementation

Use urllib/urllib2 to implement the needed part of the DAV protocol:
- share connections,
- handle user/password without requiring the user to embed them in the url.
Main motivation is that bzr should not require additional packages (like pycurl), and neither should the plugins.
Until the urllib implementation is completed, the code is unstable. The main effort currently occurs in the https://launchpad.net/people/bzr/+branch/bzr/bzr.urllib.keepalive branch, once this is completed, the plugin will be updated accordingly. Stay tuned.