Fix for rapid popPage() call

We discovered a issue with the popPage() call, when you are rapidly firing the back-button. You can get a empty page or a error in _popPage(), because there doesn't exist one. This is because there is a small delay with between the this.page.length <= 1 check and the this.page.pop() call, where there can be sended a new popPage() call thats get accepted because the amount of pages is not lowered.

Issues with this fix:
 - The _emitPrePopEvent does not return fails.
 - The performance will be a bit lower. But there will be no invalid popping
Merge pull request #202 from Tincoboekestijn/patch-1

Fix for rapid popPage() call