Make sure each mirador instance gets unique identifier

[dodget]

This PR should resolve issues observed when instantiating more than one mirador viewer on a page. It doe so by:

• Checking the page for the number of mirador instances on the page
• Using the check to create a miradorId
• Use the miradorId in the media object div, as well as the Mirador.viewer

[dodget]

Thanks @eloyer - I've refactored the block to use me.model.id based on other examples.