Image preview size

Usage:

'image' => array(
'name' => 'image',
'label' => 'Image',
'type' => 'image',
'args' => array(
'size' => 'large'
)
)
or array :

'image' => array(
'name' => 'image',
'label' => 'Image',
'type' => 'image',
'args' => array(
'size' => array(100, 100)
)
)

if you want use custom image sizes, add this filter:

add_filter( 'image_size_names_choose', function( $sizes ) {
$possible_sizes = get_intermediate_image_sizes();
unset( $possible_sizes['full'] );
if( !empty( $possible_sizes ) )
return $possible_sizes;
else
return $sizes;
} );
Cuztom_Initializer instead of Singleton (just another name)
Merge branch 'pr/218' into dev
Syntax update (more spaces)