Extract taxonomy jstree menu to a new file

The motivation behind this is to make it possible for extensions, e.g. spree_i18n, to customize the menu options when users right click on the taxon name.

I think I could go on and convert the whole admin/taxonomy.js to coffee script as well. Please let me know your thoughts on it. I'll do some research on how we could write a feature spec for the right click option on jstree.