The url helper should not prepend Backbone.history.options.root for pushState and absolute urls

I think the only case in which Backbone.history.options.root should be prepended is when the fragment is relative.

If you initialize Backbone.history with a root like "/path/" and pass a fragment like "/articles" to the url helper, you should get "/articles", not "/path/articles". If you define a root, your route definitions do not include that root.

This PR doesn't handle the case in which a root is defined without a trailing slash; defining a root like that does not seem idiomatic based on the Backbone docs.