New function for retrieving previous and next adjacent image

[solidcolour]

There are previous_image_link() and next_image_link() functions. But there are no functions that only return true and false, which would works similarly to get_previous_post() and get_next_post() but for images. It would be very helpful to have that, so we can easily to control whether to output the pagination HTML markup in image.php.

[markparnell]

Patch adds get_previous_image_link(), get_next_image_link() and get_adjacent_image_link() which return the link instead of echoing it.