Fix bug which causes step-pane content to disappear if there is more than one wizard in the page

When there is more than one wizard in the same page, due to the global (document-wise) selection of wizard elements, i.e. `$('.step-pane').removeClass('active');` and `var index = $('.steps li').index(li);`, one wizard controls will affect every other's content pane. 

It could be easily fixed by selecting elements relative to their wizard container.