Add Sina Weibo, RSS to navbar and Fix Paginator error

Please, Sina Weibo is much more popular in mainland China. Add this poor thing to let Sina Weibo user add a link on social navbar.

The RSS is added to social navbar as well.

Another **important** fix is for paginator. It resulted in incorrect URL on Octopress-2.5 (not test on other versions). When you clicked **Older** in index, the **Older** link in page 2, which expected to be `demo.com/3`, would be `demo.com/2/3`. This bug is fixed by correcting paginator in index.html (already included in this pull request).