Sharing: Reparent the Email UI to Prevent Broken Layouts

Steps to reproduce:
1. Add the email sharing button to your site.
2. Wrap your theme's wp_footer() call in <div style="position: relative"></div>
3. Click the email sharing button.

This commit reparents the sharing_email div to the body element, and caches a couple jQuery selectors
Merge pull request #1075 from jeherve/update/sharing-email-layout

Sharing: Reparent the Email UI to Prevent Broken Layouts. props @yoavf