Update instructions to build from source for Ubuntu

On 12.04, the default node version is (0.6) and we require a more recent one to install some dependencies.

The instructions were tested on 12.04 and 13.10.

I haven't tried 12.10 or 13.04, but it shouldn't be hard for users to figure out what to do.

@atnnn would you mind taking a look at it?