WebUI: Option grouping and help text in 'Debugging' tab; sort option in recording lists (upcoming/finished/failed)

Two parts to this one, then:

1. Some tidying up of the 'Debugging' tab (tvhlog.js) - options are grouped to be consistent with other screens, but I've also added some placeholder help text and an accompanying image. 

If someone can give me some more explanation of the debug options, I'll finish off the text - but better a placeholder than a null response IMO.

![debuglog](https://cloud.githubusercontent.com/assets/6093716/3083552/dcc6f350-e4e4-11e3-849b-cf9f10a8ad85.png)


2. Switch from remote to local sort for the recording lists (dvr.js) with addition of per-column sort capability (e.g. sort your recordings by name, duration, channel, etc. and not just by the default date). 

The downside I see of this is that local sorting might be fractionally slower because it's browser-based (I believe). However, because we're paging the entries (< 100 per page) then this isn't noticeable - it only sorts the current page, not all entries. I don't know it would work so well on a single page of 100k EPG entries, though :-) 

![namesort](https://cloud.githubusercontent.com/assets/6093716/3083537/a4c9b230-e4e4-11e3-96c7-44bc0734b2cc.png)
![durationsort](https://cloud.githubusercontent.com/assets/6093716/3083539/a8265050-e4e4-11e3-87fa-d8d9dc01db94.png)
![channelsort](https://cloud.githubusercontent.com/assets/6093716/3083541/acc1b064-e4e4-11e3-96c0-0d16b8ae374b.png)