fix for secondary issue in #9

Added conditional bindings to `core-icon-button` for `menuButton` to position it to the right of content when `rightDrawer` is enabled.  This was to address a request from #9.

I didn't want to be too heavy-handed and force the button to be pushed to the right by adding a `<div flex></div>` before the `core-icon-button`, because it would prevent flex content from filling the entire width when using core-scaffold (since using flex in the content div would only split the width with  `<div flex></div>` inside core-scaffold).  This way is more flexible (pardon the pun) without having to modify `core-scaffold` to get it to do what you want.  Example here: http://jsbin.com/saweqesozuki/1/edit

I considered adding a `rightMenu` attribute, but it made more sense to tie this behavior to `rightDrawer`.

Also, added some missing documentation and new documentation for this function.