Fix for Back button bug with two cards / CSS formatting update / Added optional "Cancel Button" functionality

* [Bug Fix] - Pulled out the enable/disable back button functionality so that it works properly when there are only two cards in the wizard. -- "the Back button bug"

* [Refactor] - Updated some of the CSS formatting

* [New Feature] - Added a Cancel button in the footer (this is NOT enabled by default and must be turned on with the "showCancel" argument on initialization.. see example below)

Example:

    var wizard = $('#wizard').wizard({ showCancel: true });

Love the project! Keep up the great work!

( To make the review easier, you can hide the whitespace changes: https://github.com/bryancallahan/bootstrap-application-wizard/compare/amoffat:master...bryancallahan:master?w=1 )