Use custom HttpConnector when authentication

I'm trying to modify ghprb-plugin so that it can work behind authentication proxies.
I added a HttpConnector implementation to ghprb-plugin:

```java
public class HttpConnectorWithJenkinsProxy implements HttpConnector{
    public HttpURLConnection connect(URL url) throws IOException {
        return (HttpURLConnection)ProxyConfiguration.open(url);
    }
}
```

Since all `GitHub` constructors are `private`, there is no way to call `#setConnector` before `GitHub(String, String, Sttring, String)` constructor calls `#getMyself`, nor to extend `GitHub` class.
So I added `HttpConnector` parameter to `connect*` methods, except `connectToEnterprise` which does not require proxies in most cases.