Disables itself when used in a file:// page

This change was made because when somebody tries to use a page with instantclick.js via a file:// url, all the links sort of glitch out. They require you to click twice and don't add any speed improvements.
Moved location.protocol check to supported variable
Merge pull request #43 from wtfaremyinitials/master

Disables itself when used in a file:// page