Added proxy support with http_proxy configuration

Sites sections of praw.ini now handle the optional directive
'http_proxy'.
Example
[reddit.com]
http_proxy:127.0.0.1:8000

Available URI are the ones handled by the Requests python module.
Proxy authentication would work with:
http_proxy:user:password@host:port

All tests passing (regular tests as well as python setup.py test -s
praw.tests.OAuth2Test