SocketIO event filtering

[daffl]

This pull request implements event filtering for SocketIO. Letting you filter events based on the connected client information. For example the following only dispatches the updated event if the authorized user is in the same company than the updated todo:

app.configure(feathers.socketio(function(io) {
  io.set('authorization', function (handshake, callback) {
    // Authorize using the /users service
    app.lookup('users').find({
      username: handshake.username,
      password: handshake.password
    }, function(error, user) {
      if(!error || !user) {
        return callback(error, false);
      }

      handshake.feathers = {
        user: user
      };

      callback(null, true);
    });
  });
}));

app.use('todos', {
  update: function(id, data, params, callback) {
    // Update
    callback(null, data);
  },

  updated: function(todo, params, callback) {
    // params === handshake.feathers
    if(todo.companyId === params.user.companyId) {
      // Dispatch the todo data to this client
      return callback(null, todo);
    }

    // Call back with a falsy value to prevent dispatching
    callback(null, false);
  }
});

On the client:

socket.on('todo updated', function(data) {
  // The client will only get this event
  // if authorized and in the same company
});

Closes #44, closes #48.

[daffl]

This is probably the biggest feature for the next release so we should make sure that this works nicely.

/cc @Glavin001, @ekryski

[Glavin001]

This will be great! I'll try and test it out tomorrow.

[daffl]

So #51 is based on this PR and implements the whole thing for Primus which I think should finalize this functionality.

[daffl]

If there are no comments or concerns I'll merge this one and #51 in tonight so that we can get a 0.4.0 out this week.

[Glavin001]

👍 I looked over all of the changes and everything looked good. I have not had a chance to personal test it, but I didn't notice any missing Tests either and Travis CI is still happy.
Merge away! This is going to be an awesome feature for 0.4.0