Add options for size and image defaults

This sets up options so you can pass a default image path or a default image size to the picture_tag method.  Examples:

<%= picture_tag(p.url, {:alt => p.alt_text, :prefix_size => true, :default_size => :large}).html_safe %>

would load the large image size by default before executing the media query specific javascript

and...

<%= picture_tag(p.url, {:alt => p.alt_text, :prefix_size => true, :default_image => "/images/test3.jpg"}).html_safe %>

will use /images/test3.jpg as the default before executing the javascript