refactor($rootScope): consistently use noop as the default $watch listener

[jbedard]

By doing the isFunction(listener) check before delegating to $$watchDelegate it removes that check from some $$watchDelegate methods.

[petebacondarwin]

I think there are other opportunities here:

angular.js/src/ngMessageFormat/messageFormatInterpolationParts.js

Lines 125 to 127 in 5838017

	if (isFunction(this.listener)) {
	this.listener.call(null, result, newExpressionValues === oldExpressionValues ? result : this.previousResult, this.scope);
	}

and here:

angular.js/src/ngMessageFormat/messageFormatSelector.js

Lines 69 to 71 in 5838017

	if (isFunction(this.listener)) {
	this.listener.call(null, newMessage, newMessage === oldMessage ? newMessage : this.lastMessage, this.scope);
	}

[Narretz]

@jbedard do you want to incorporate @petebacondarwin suggestions? If not, I will merge

[jbedard]

I did those suggestions (but please double check!). I think it's ready to merge.