Trigger initial change after object is linked to DOM element

Calling fixNavigationButtons after dom element has reference to wizard object make ir possible to call api methods in onTabShow event handler.

Demo with current master
http://plnkr.co/YcVPEGHAaYJrZZ7fIITQ

And demo with fixed bug
http://plnkr.co/3PqHm9vqBvFVe7ubXUef