Added slider control to make Web UI poll interval configurable

Possible fix for #1711

To implement resetting of the polling interval, I used a simple HTML5 slider control rather than a more elaborate jQuery knob widget, as I wasn't sure if introducing an additional plugin would be acceptable (e.g. [jQuery Knob](http://anthonyterrien.com/knob/)). I can change that, though, if something like that would be preferable. 

![screen shot 2014-05-17 at 10 24 58 pm](https://cloud.githubusercontent.com/assets/30317/3006781/c145bb28-de4c-11e3-8fbf-d2a34d5b7559.png)

I set the minimum interval to 2s and the maximum to 20s. Is degree of configurability enough?

Also, I used `sessionStorage` in order to persist the chosen polling interval upon navigation from tab to tab.

Hope this is the direction you had in mind...