Add automatic proxy handling for WinHTTP and BITS

[zooba]

Turns out the default "automatic" behaviour isn't automatic enough, so add the manual steps required to handle various configuration approaches.
Fixes #128

[zooba]

I've tested this enough to confirm that bad proxy settings do get loaded into the WinHTTP process now, and so causes a failure. If anyone has a working proxy handy that they can test with, it's hopefully not too difficult to build this and run a py list --online -vv to see if it works (and if so, a py install --download ... to validate that the BITS code also works).

[Jose-albino]

If you can provide a msi.exe build and I can test

[zooba]

We're still in beta, so the next available build will just be our next release. Probably this week some time.