add custom base url support

This add support of costom base url.

In our country china, for some reason you knew, origin gravatar url offen get blocked or very slow, so there are many alternative CDN Provider give us a extra option to resolve this matter.

For example, we can use http://gravatar.duoshuo.com/ or http://9429127371.a.uxengine.net/ istead of origin http://www.gravatar.com/, like [this image](http://gravatar.duoshuo.com/avatar/c197954ab376347f119efb5ce1e6de1d?s=96&d=mm&r=g). it's just a reverse proxy so I think it's safe enough.

So if it is convenient for you, could you accept this request?

Thanks!