Allow calling all master methods from worker
Allow calling all master methods from worker

Problem: Certain master methods like spawn() were previously
not possible to call from the worker, because they do not
accept a `worker` parameter as the first argument.

This patch allows calling a master method without providing a
worker id like so:

master.call(false, 'spawn', 1);

Fixes #123

koudelka/cluster,mauricionr/cluster,LearnBoost/cluster,LearnBoost/cluster,mauricionr/cluster,koudelka/cluster