Add proxy support for HTTP requests

[cyberbit]

This adds explicit proxy support for API endpoints using standard HTTP requests. In many cases, requests simply reads the HTTP_PROXY and HTTPS_PROXY environment variables. However, in my case, I have (closed-source) software that treats HTTPS_PROXY differently than everything else, so I cannot set it without causing errors. This change allows me to patch my unusual environment.

Usage:

pb = PushBullet('xxx', {'http': 'http://10.0.0.1:8100', 'https': 'https://10.0.0.1:8101'})

The format and behavior is exactly as defined here: http://docs.python-requests.org/en/master/user/advanced/#proxies