If ommited or null, delay for refered methods will default to 0. This
will make setTimeout and setInterval methods to behave as expected by
[HTML5 specs](http://www.w3.org/TR/html51/webappapis.html#timers):
"Let timeout [delay] be the second argument to the method, or zero if the
argument was omitted."
This commit also fixes an issue with tick() being called without arguments,
that causes the scheduler to break and stop working after this call.
