Small typo
Broaden the coverage of isFirstSlide test

If we:
- visit a vertical slide

and then:
- go to the first slide

the function will return **false**.

Added a test to isLastSlide too just to be sure.
kill-yank mistake
fixed a problem in the function isFirstSlide

when visiting a vertical slide and then going back to the first slide, the function would return false.
made it more fail proof by checking the indices directly.
== --> ===
Merge branch 'fix-isFirstSlide' of https://github.com/fabianorosas/reveal.js into dev