refactor click rebinding

Add onTabChange Event

Prevent double-firing of events

Added an onTabChange event, which fires an event
whenever a tab is changed. The callback function passes
tab, navigation, oldIndex, newIndex, in that order.

fix navigation buttons selectors

Update README.md

removed > as it breaks styling of tabs #22

Only look for direct children

By only checking for ```<ul>```s that are a direct child of the root wizard element (and only tabs that are part of that navigation) we're able to nest a second tab group within the main ```#rootwizard``` div.

This may or may not fix nested wizards, I haven't tested that. However it *does* fix [the scenario](https://github.com/VinceG/twitter-bootstrap-wizard/issues/11#issuecomment-16353032) I commented about in issue #11.

Like I mentioned in my comment on that issue I'm not entirely sure how closely related a "wizard" is to a "vanilla" bootstrap tab group (at least when it comes to nesting them), but this could well do the trick.

Adding Custom Styles example

Fix for the problem described on Issue 4

Added the index of the clicked tab to the 'onTabClick' event. To get in this point what is the clicked tab (before that we only know about the actual tab)

Exposing methods