persisting polling interval setting with localStorage; added tick marks ...

...with numbers to HTML5 slider element

1. Did some research on adding tick marks to the slider element. It does exist, but unfortunately is only supported by Chrome and IE10. So I ended up using a simple `ul` to add the tick marks and numbers:
![screen shot 2014-05-18 at 4 03 03 pm](https://cloud.githubusercontent.com/assets/30317/3009045/b5f916fe-dee0-11e3-8fe3-87aaba456c10.png)
Also the "current interval" label is updated dynamically (on `mousemove`) as the slider is dragged.

2. Changed `sessionStorage` to  `localStorage`.