Fullpath argument for annotation tags

Purpose:

In some instances, the mobile or tablet versions of a site do not share the same url structure as their canonical counterpart. In this pull request I expose an optional `:fullpath` argument for the `ress_annotation_tags` method.

Simple example:

Say I visit `www.mysite.com/confirmed` but there is no mobile equivalent url available (say: `m.mysite.com/confirmed` does not exist). The mobile or tablet equivalent might be `m.mysite.com/cant_confirm`. So we'd do that with:

    ress_annotations_tags(:fullpath => '/cant_confirm')

This will create:

    <link href="http://m.mysite.com/cant_confirm" id="mobile" media="only screen and (max-width: 640px)" rel="alternate">

RESS states that html pages should contain all their devise equivalent urls in the head of the html page. In some cases, the url structures are not all the same but you still need to list the other equivalents. This takes care of that.