WIP: torrent cmd: Initial work on supporting 'set'

You can now query and set values using the following syntax:

> torrent set upload_limit

> torrent set upload_limit 100

TODO:
* Only support for setting numerical values currently...
  - JSON would encode nums as strings, so we numify.
  - Problem is values that aren't numerical... Gah.
  - Should probably be fixed in Transmission::Torrent