Issue title: onLeave index value incorrect
 Issue body: Hi,

if you have 5 sections and scroll from your first section to the next section with 

"$.fn.fullpage.moveSectionDown();"

or just scroll you will get index number "1"

if you then scroll up (to the) initial section you get index number "2"

am im doing anything wrong? 

`onLeave: function(index, nextIndex, direction){
                var leavingSection = $(this);
                alert(index)`
### Versions

OS X latest and latest browsers tested safari, chrome, ff.

 Comments: 
 Comment 0: Please provide an isolated reproduction in jsfiddle or codepen.
[The demo page](http://alvarotrigo.com/fullPage/examples/callbacks.html#secondPage) is working as expected as well as [the demo in codepen](http://codepen.io/alvarotrigo/pen/XbPNQv).

Closed until an isolated reproduction is provided.

{'source': 'GithubIssues', 'repo_name_with_owner': 'alvarotrigo/fullPage.js', 'issue_id': 157900380, 'repo_stars': 34145.0}