Update urllib3 to 14b8945
Fix for new proxy API.
Cache manager for proxy connection.
Added proxy support to feature list.
Added entry to AUTHORS.
Add proxy_headers functionality.

This brings us in line with urllib3's fancy new magic for sending
headers to HTTP(S) proxies.
Merge pull request #1515 from schlamar/https-proxy-2.0

[2.0] Https proxy support