Update to README installation instructions

Hi,
I have just installed your code on a Raspberry Pi running debian.
I had to do things slightly differently to your instructions (just the odd sudo here and there, and moving printrun).
This pull request worked for me, although I still haven't got forever working - I got errors with the @0.9.2 on the npm command, but it installed just asking for 'forever', but then the forever start command failed with
/usr/lib/node_modules/forever/lib/forever.js:404
  monitor.send(JSON.stringify(options));
          ^
TypeError: Object #<ChildProcess> has no method 'send'

This is why I added the instructions to start it without using forever.

I haven't tested it properly, so may need to make more changes, but thought I'd note these down before I forget.

Regards

Graham.