sip._auto() callback called when returned stream ends

[jackrobertscott]

Like gulp, allow streams to be returned so that the it is unnecessary to call the actual callback function.

tree[name] = (this.tasks[name].deps || []).concat([function(cb, res) {
    // TODO check emit 'end' event for cb
    // 'res' contains the callback results from dependent functions
    this.tasks[name].fn(cb, opts, res);
  }.bind(this)]);

To do:

• Check if stream
• Attach cb to end event
• Prevent multiple cb calls