Metaboxes are initialized too early for some use cases

For my current project, I needed to setup a select that pulls in custom taxonomy terms.  "Use a taxonomy_select" you may rightly say, but for this use case I don't want the term added to the post. I just need it for reference. 

In the process of setting up this select with the tag terms, I used this code:
``` php
		$tags = get_terms( array('shopp_tag') );
		$tag_options = array();

		foreach($tags as $t) {
			$tag_options[] = array('name' => $t->name, 'value' => $t->name);
		}
```

I then setup a new select with the `$tag_options` array:
``` php
				array(
					'name'     => __( 'Special Tag', 'cmb' ),
					'desc'     => 'This is a special tag.'
					'id'       => $this->prefix . 'special_tag',
					'type'     => 'select',
					'options' => $tag_options,
				),
```

However, it didn't work.  When I dug deeper, I realized `get_terms` was returning "Invalid taxonomy" because `cmb_meta_boxes` is called before init, the preferred hook to register taxonomies. 

While this is one use case, I think there are plenty of other potential conflicts here that could be perfectly valid use cases.  A selector that allows you to tie a post type to a page, for example would also fail. 

Given this, I'm suggesting we run the metabox setup on the `wp_loaded` hook.  I've tested this and it's late enough that it works fine, but still early enough it doesn't impact other processes. 

The only thing I haven't tested is metaboxes on the front end.  I don't have any projects that utilize this at the moment.