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

- 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
Adds support for loading child or sibling view controllers from nibs

- Controller nib is assumed to have same name as controller class
- Controller placeholder view should have a restoration identifier
  beginning with 'placeholder'
- Child controller nib should have the controller's view as the first
  view object (as is typically the case)
Don't apply placeholder's constraints to replacing view for constratints between the placeholder and it's subviews
Update readme to include link to wiki for nib loading helpers
Update CHANGES.markdown to include note about nib loading helpers
Set UIKit deployment target to iOS 6.0 at the root of the project
Merge pull request #65 from pivotal/NibLoadingHelper

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