Improve performance of Downloader.

• Update HttpClient to 4.3.1 and rewrite the code of HttpClientDownloader #32.
• Use gzip by default to reduce the transport cost #31.
• Enable HTTP Keep-Alive and connection persistence, fix the wrong usage of PoolConnectionManage r#30.

The performance of Downloader is improved by 90% in my test.Test code: Kr36NewsModel.java.

Add synchronzing API for small task #28.

        OOSpider ooSpider = OOSpider.create(Site.me().setSleepTime(100), BaiduBaike.class);
        BaiduBaike baike = ooSpider.<BaiduBaike>get("http://baike.baidu.com/search/word?word=httpclient&pic=1&sug=1&enc=utf8");
        System.out.println(baike);

More config for site

• Http proxy support by Site.setHttpProxy #22.
• More http header customizing support by Site.addHeader #27.
• Allow disable gzip by Site.setUseGzip(false).
• Move Site.addStartUrl to Spider.addUrl because I think startUrl is more a Spider's property than Site.

Code refactor in Spider

• Refactor the multi-thread part of Spider and fix some concurrent problem.
• Import Google Guava API for simpler code.
• Allow add request with more information by Spider.addRequest() instead of addUrl #29.
• Allow just downloading start urls without spawn urls extracted by Spider.setSpawnUrl(false).