Fix reloadData bug causing no refresh when content changes

`reloadData` has a bug introduced in 7d91f780875c28985891d4681b772506a8a2e8e1 which causes new content not to refresh

**Steps to reproduce:**
* Display an SYPaginator with several pages of images
* Alter the data source, such that `-paginatorView:viewForPageAtIndex:` will return new pages/content
* Call `[paginatorView reloadData]`

**Expected result**
* SYPaginator updates to display the new set of pages

**Actual result**
* SYPaginator does not update. viewForPageAtIndex is not called

This PR bypasses that issue by allowing "non-public" calls to _setCurrentPageIndex to request a content refresh. Feedback appreciated @dorshorst