WebUI: Change torrent model

Improvements:
- torrents data request response contains only changed data. When all
torrents paused and there are no uploads response will look like this :
{ "rid" : 12 } All requests responses can be shortened similarly.
- added functions, which may be used to reorder and hiding columns in torrents table
- new columns can be easily added to torrents table
- improved responsiveness of the WebUI