Note: After saving, you have to bypass your browser's cache to see the changes. Internet Explorer: press Ctrl-F5, Mozilla: hold down Shift while clicking Reload (or press Ctrl-Shift-R), Opera/Konqueror: press F5, Safari: hold down Shift + Alt while clicking Reload, Chrome: hold down Shift while clicking Reload.
var catpurge = {
  
  init: function() {
    mw.util.addPortletLink( "p-cactions", "javascript:catpurge.purge()", 'CatPurge', 'ca-catpurge', 'Purge all images in this category', null );
  },

  purge: function() {
    var api = new mw.Api();
    api.post( {
      action: 'purge',
      generator: 'categorymembers',
      gcmtitle: mw.config.get( 'wgPageName' ),
      gcmnamespace: '6',
      gcmlimit: '500'
    })
    .done( function( data ) {
      mw.log( 'API result:', data );
    } )
    .fail( function( error ) {
      mw.log( 'API failed :(', error );
      alert( 'API failed: ' + error );
    });
  }
};


if( mw.config.get( 'wgNamespaceNumber' ) === 14 ) {
 $( catpurge.init );
}