Make sameOrigin a public method

This change allows other JS in your app to use the sameOrigin method
provided by page.js by calling `page.sameOrigin(href);`.

I’ve found it useful to use this function in my own code and it’d be
handy to just use the one provided by page.js rather than copy/paste it
out and load it into memory twice in two different JS libraries.
Merge pull request #124 from kethinov/exposing-sameOrigin

Make sameOrigin a public method