Support for multi-users!

This NodeJS implementation is the first one to receive the MU support. It allows you to run multiple instances of PB on a same server, with all configuration in the same ~/.photobackup file.
For that, we added an optional parameter to the init and run commands, called username, that specify which username to use. If none is given, the default (and already known) behaviour takes place. Hence, we ensure complete backward compatibility.