overlay is now optional

call `overlay` function if you want the tour to have overlay
by default everything remains clickable
add Tour.react function

Tour.react will temporarily hide the current step and then re-display it
after the timeout. It allows tour to react to user actions - especially
those that would reposition the referenced element.
add index param to Tour.play

you can now start tour from any step by passing a parameter to play
it not parameter is passed tours starts from the first step on initial
play, and from the 'next' step if play is called more than once
Merge branch 'react'
Release 0.6.0