Function: addSubview
Adds a Barefoot.View as subview of this view. Sets the parentView property
of the subview to this view.

Parameters:
(Barefoot.View) subview - <Barefoot.View> to add as subview to this view
function addSubview(subview) {
	debug('add subview `%s`', this.toString());
	if(_.isUndefined(this.subviews)) {
		this.subviews = [];
	}

	subview.parentView = this;
	this.subviews.push(subview);
}