Gallery radio buttons

A stab at adding the option to override the default [gallery] tag behavior which displays all images uploaded/attached to the post. The user can check "Use alternate image id's" and pass comma separated ids of any images to override the default [gallery] behavior. This is using the WordPress shortcode_atts_{gallery} filter to manipulate the gallery output.
![screen shot 2014-08-22 at 2 31 32 pm](https://cloud.githubusercontent.com/assets/1243118/4017033/9af58ecc-2a3b-11e4-9230-c7c602b11916.png)

I think a full shortcode might be easier for people to understand here.