Added example to gh-pages.

You can no longer preview the example via htmlpreview. In Chrome you see an error: `Refused to execute script from 'https://raw.github.com/zamiang/jquery.poplockit/master/src/poplockit.js' because its MIME type ('text/plain') is not executable, and strict MIME type checking is enabled. `

The recommended way seems to use gh-pages, so this adds the example there. Will make a separate PR to change the link in the README.