Add select/deselect methods to the REST api

This methods enables to start/stop downloading a file from a torrent through the
REST api.

It's my first time using javascript, node et all. So please sorry in advance if this is not really the style. In particular, I was unsure about the console.log I added, as it's never used in the server/index.js file.

Please let me know what should I change or feel free to change it yourself :-)



Thanks!
Rodrigo