added support for shortcode nesting

Added support for nesting other shortcodes inside of the box and column shortcodes.

Also -
It looks like the usl_user shortcode is broken. The example says you can include an attribute of value1="first_name". Should that be "user_firstname" instead?

Maybe consider adding transparent as a color option for the box and column shortcodes? I can see users wanting that.

Everything else looks great!