Add method that allow get currentPageIndex value

[TiagoSAmaral]

Hi!

I liked this project, and start use in personal project. But, sometimes i need get the number of page from viewed PDF. But PDFViewController is has not this possibiliti. So, i created a new method which allow only get the current value of page (currentIndexPage), whithout change any other feature. I hope that usefull. I hope did usefull.