issue/1782-reader-swipe-indicator

Fix #1782 - uses a styled undo bar to show a tip letting the user know when they can swipe through posts in the reader detail view. Note that this has been written in a way that allows for additional tips in the future.

![reader-tip](https://cloud.githubusercontent.com/assets/3903757/4935988/7ce2b820-65b3-11e4-82f9-436b430f677e.png)