Nibs can be used to load other nib-based view classes

We thought it would be best to wait on merging this until we are working on 0.1.0, since this requires bumping the min deployment target to iOS 6.  

- Placeholder view needs to be set to the view's runtime class
- restorationIdentifier must start with "placeholder"
- Nib must have same name as view class
- Could possibly be made more abstract to work with other base classes
- Bumps deployment target to iOS 6 as this supports preserving autolayout constraints