增加IP代理认证功能

[jwlyn]

以前的代理功能不能提供认证。但是从网上买来的代理一般都是有用户和密码认证的。
用法：

 site.setHttpProxyPool(Lists.newArrayList( new String[]{"userA", "pswA", "103.27.125.250", "8000"}, new String[]{"userB", "pswB", "103.24.125.251", "8000"} ), false); 

[robin-obrien]

@oxf1
How can I use this commit in my code, it is exactly what I need!

[jwlyn]

@robin-obrien the author of webmagic has not accept my patch. You can refer to my commit 9ff0ca2

[robin-obrien]

@oxf1, I downloaded the code and merged it manually, well done. It works very well. Thanks!
@code4craft, you should consider adding this commit, excellent work

[code4craft]

Merged on 83c27eb

Thanks for your contribution!
I changed some code due to conflict, you can check this commit.