Hey Rolle! Check out this link: http://cjsharp.com/blog/2013/04/15/how-to-add-valid-elements-like-iframes-to-tinymce-in-episerver-7/

The only minor change to that post is what Linus Ekström mentioned in the comments about ServerSideOnly.

So for your case, the class would look like this:

[TinyMCEPluginNonVisual(ServerSideOnly = true, PlugInName = "TinyMceExtendedValidElements", AlwaysEnabled = true, EditorInitConfigurationOptions = "{ extended_valid_elements: 'iframe[allowfullscreen|src|frameborder=0|alt|title|width|height|align|name]' }")]
public class TinyMceExtendedValidElements
{
}

You can add that anywhere in your project.

Then add the configuration in web.config:

<episerver>
	... some configuration ... 
	<tinyMCE mergedConfigurationProperties="valid_elements, extended_valid_elements, invalid_elements, valid_child_elements" />
</episerver>

And that's it!