urllib3: added HTTPS proxy support (shazow/urllib3#139)
Fix for new proxy API
Cache manager for proxy connection
Cache proxy manager by URL and not scheme

So we use the same manager if there is the same proxy for multiple
schemes.
Merge branch 'new-urllib3-api' of github.com:schlamar/requests

Adds HTTPS proxy support
A Session object now maintains a response hook property for the most recent request