Allow multiple .steps and .btn-prev/.btn-next buttons with each wizard.

Useful especially for large forms where, for example, you'd want to have next/prev buttons and/or steps at both the top and the bottom of the step content.  The CSS targets the next/prev button in the wizard instead of specifically in the .actions div.  The next/prev buttons target each individually and iterate over them, making any necessary changes to state.  And the .steps divs are targeted individually to find the correct index and activate the active step.