Metabox API

This implements what I'm calling the Largo Metabox API. It implements three key functions for conveniently creating new meta boxes (and content/fields within those boxes):

largo_add_meta_box() : basically an extension of WP's own add_meta_box()
largo_add_meta_content() : a tool for adding content/fields to meta boxes created using largo_add_meta_box()
largo_register_meta_input() : a quick way to get any input elements from the above markup quickly into post meta fields.

This is a first pass at the API architecture. I can pretty easily envision some extra features, such as adding weighing to largo_add_meta_content() to offer more control regarding *where* in a particular pre-existing meta box new content is placed, or implementing some validation functions and arguments to make registered inputs a little smarter (though they're certainly not any *dumber* than they were before).