Adding a step now returns the Tour object

Returning the Tour object allows users to chain adding steps
Update shepherd.coffee

Return Tour after adding a step
Updated all files with Tour::addStep now returning Tour.
Merge branch 'master' of https://github.com/ilanbiala/shepherd

* 'master' of https://github.com/ilanbiala/shepherd:
  Updated all files with Tour::addStep now returning Tour.
  Update shepherd.coffee