Exposes toolbar on Firepad and makeButton_ on RichTextToolbar

These are two minor changes which allow easier modification of Firepad's built-in toolbar by developers wishing to expand functionality. The first is moving `Firepad.addToolbar_`'s `RichTextToolbar` instance to a place where developers can control it (i.e. the Firepad object). The second is moving `RichTextToolbar.makeElement_`'s `btn` method to the `RichTextToolbar` prototype as `makeButton_` so it can be used as a utility function for developers to generate Firepad style buttons to append to `RichTextToolbar._element` if they wish to add new buttons to the toolbar.