Patch behavior for added callbacks when DOM is already ready

See #34

When loading scripts asynchronously, we noticed inconsistent behavior where the initial page load fired all of our handlers, but subsequent Turbolinks page loads did not fire any handlers.

This fix calls the callback immediately if the DOM is ready and binds to the page load event for subsequent Turbolinks page loads.
Merge pull request #45 from neilsarkar/master

Patch behavior for added callbacks when DOM is already ready