Missing icon for the context menu in built version.

[f1ames]

Hello again,
while the icon for the context menu is visible in the dev version of the plugin, for the built version of CKEditor all icons are combined into a sprite so the path for the context menu icon (https://github.com/iametza/ckeditor-html5-audio/blob/master/html5audio/plugin.js#L73) is broken:

You could simply use icon: 'html5audio', so it references the icon from the plugin definition (https://github.com/iametza/ckeditor-html5-audio/blob/master/html5audio/plugin.js#L4).

Tested in Chrome 51.