Allow using a proxy

[tim95030]

I added this in a way that nothing is affected unless you define a proxy with PGoApi.set_proxy or by passing in a proxy to the PGoApiRequest method. I defaulted it to None and it uses the standard requests library if it is none.

[tejado]

Can you please check why travis-ci is failing?

[tim95030]

I have been working on that. It appears to be an issue with Python3, but I don't know how to fix it yet. I haven't had time to figure this out at the moment. I should be able to this weekend.

[DSlayerMan]

The current requests library supports proxies and socks. http://docs.python-requests.org/en/master/user/advanced/#proxies

[tim95030]

Ok. I will fix it up to use requests[socks] then. Didn't realize they added that.

[beeblebox]

@tim95030 check your fb messages

[ndurchx]

I think, the authentification should also use the proxy.

[tim95030]

@ndurchx What do you mean? All requests use the proxy with this change. You turn on the proxy and all requests use it. If you mean it should support proxies with authentication, then that was not my goal and can be added later separately.