Closed Position + IBInspectable Support

• IBInspectable support for PullerDrawerViewController settings.
• Formally added / fixed edge cases for a 'closed' drawer position.
• Added "initialDrawerPosition" option so the drawer starts in the position you'd like.
• Added a programmatic example for initialDrawerPosition in AppDelegate.
• Fixed a constraint issue in the Interface Builder example.
• Made the PulleyPosition.all accessible outside of the module