Add Libscore badge & syntax highlighting to README

Hi there,

I just checked your project's score on Libscore, and was very impressed by how well it did: http://libscore.com/#$.iframeHeight (Libscore only scans the __homepage__ of the top 1'000'000 on the Web).

So I thought it might be cool to add a Libscore badge to the readme. I also took the opportunity to add syntax highlighting to all the code samples: not a big change, but cool-looking anyway. Result: https://github.com/ThibWeb/Iframe-Height-Jquery-Plugin/blob/756e0b22d519211210b3d3abe31d6203e94e6748/readme.markdown

Cheers!