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 JS / CSS formatting to be more consistent with the plugin as a whole

* [New Feature] - Added a Cancel button in the footer which has the same functionality as the close button (this is NOT enabled by default and must be turned on with the "showCancel" config param on initialization.. see example below)

* [New Feature] - Added a configuration to allow users to click between the cards on the nav in any order he or she wishes (this is NOT enabled by default and must be turned on with the "forceNavSequence" config param on initialization.. see example below)

* [New Feature] - Added a "closed" event on Wizard which fires after a user closed the Wizard.

Example:

    var wizard = $('#wizard').wizard({ showCancel: true, forceNavSequence: false });

Love the project! Keep up the great work! (Note that this does not contain the changes for the minified version. Please let me know if you would like me to include this.)

( 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 )