Added two options to add transition with specific duration between steps

Can use transition between steps.

Options :
- transition :
   - 'fade' : FadeOut/In
   - 'slide' : SlideUp/Down
   - default -> 'hide'

- duration (in ms) :
   - default : 0

Example :
$( '#my-form' ).stepy({transition : 'fade', duration: 400});