Allow HTTP_PROXY settings to use username and password

[winstonwolff]

I am using an http proxy service (FIXIE) on Heroku that requires a username and password. I've added 4 lines that will pass along the username and password from the HTTP_PROXY environment variable. Formerly just the hostname and port were used.

Would you like to merge this into your master?

[rubyjedi]

Hi, thanks for the PR contribution.

I'm not sure what's going on with Travis CI at the moment to suggest it's broken (your patch looks perfectly fine). Let me hammer on Travis configs a bit, and once I have it sorted out, I'll pull this into master. Thanks!