Extension:GuidedTour/Back button discussion 2014-06-05


 * Based on how new tours are defined with non-linear transitions (NLT)
 * Looked at how lib.Step has transitions. Can assign transition/next. Have a back callback, with same functionality as back.
 * Can we use .back analogously to .next?
 * This part is okay


 * Start with .back and maybe revisit later.
 * .shouldShowBack
 * Interimly, if back only sometimes would work on particular step, don't include it.
 * No shouldShowBack for now


 * If .back is called, show the back button.
 * Do the same thing for .next, but accept action: 'next' (deprecated) for backwards compatibility. If action: 'next' is not specified, generate the next button.
 * No action: 'back'


 * No auto-wiring for now (generating back button if there was previously a next)