Optimist is no longer maintained. It has not been updated since 2014,
even for basic security updates. Thus this change replaces it with its
spiritual successor, yargs, which is still actively maintained.
This is also a first step to removing peerjs' dependence on an outdated
version of minimist that has known security vulnerabilities.
PeerServer is now instance of express application - this allows to combine it with other express applications.
Listen is deferred - because it now method from express (you can call it later and supply callback for start event).
Constructor now not using `new` (because we mixing in methods to express app)
Closes#36
