Delegate pageEndScrolling: crash fix

EAIntroView crashes when `intro:pageEndScrolling:withIndex:` is implemented by the delegate. This crash results from the Pages array going out of bounds.

#### Reproduce: ####
1. In the example project, implement  `intro:pageEndScrolling:withIndex:` with a blank body
2. Run the project, scroll beyond the last page of **any** demo

App crashes with array out of bounds

#### Culprit: ####
The `setCurrentPageIndex` setter never actually updates `_currentPageIndex`. Thus, when `checkIndexForScrollView` attempts to save the day by decrementing the index -- no change persists

#### Fix: ####
1. Update currentPageIndex using its ivar instead of the public setter
2. Prevent delegate method `pageEndScrolling` from firing a second time with the last page's index