Updated Preview link to the correct preview site

I am assuming the correct preview page for the layout is http://hexo.io/hexo-theme-landscape/ so have updated the link in the readme file.

This change should resolve https://github.com/hexojs/hexo-theme-landscape/issues/6
Merge pull request #7 from jr0cket/patch-1

Updated Preview link to the correct preview site
Add Facebook Open Graph settings
Prevent text size adjusting on Safari on iOS. Fix #9