Add ability to set metabox priority

Adds the ability to set the priority on metaboxes. 

Example:
```

add_action( 'admin_init', function() {

	$titan = TitanFramework::getInstance( 'titan-test' );

	$box = $titan->createMetaBox( array(
		'name' => 'Meta Box 1',
		'post_type' => 'post',
		'priority' => 'low',
	) );

	$box->createOption( array(
		'name' => 'My Option',
		'id' => 'my_option_1',
	) );


	$box = $titan->createMetaBox( array(
		'name' => 'Meta Box 2',
		'post_type' => 'post',
	) );

	$box->createOption( array(
		'name' => 'My Option',
		'id' => 'my_option_2',
	) );

});
```