Support changing to a specific panel

Implemented -[MYBlurIntroductionView changeToPanelAtIndex:].
Supports both text directions, as far as I'm able to test.
Unclear whether the original intention of the function was to take the
absolute index of the panel to display or the index relative to the
text direction.
Merge pull request #6 from Funnyman22/master

Support changing to a specific panel