Reuse .mercury-modal and .mercury-modal-overlay elements

If multiple Mercury modals are opened, each one creates a .mercury-modal div and a .mercury-modal-overlay div and appends them to the body. When a modal is hidden, it is reset, and all its content is cleared, but the divs are left. That leads me to believe that the original intention was to reuse the divs for new modals. This pull request implements that reuse.