Add ability to select which head.js file to use

I wanted the ability to use the full head.js file (head.min.js) rather than the asset loader version (head.load.min.js), this change adds the option in the admin settings and adds the appropriate file into the js folder.

I initially thought of using a check box, but instead went with a textbox so other versions of head.js can also be put in the js folder and then loaded instead, if the user so wishes.