Adding dequeue from Nib

Adding methods to dequeue cells and supplementary views from Nib, in addition to classes.
officially indicate 10.8+ compatibility
Merge pull request #96 from jwilling/add-compatibility-note

Add compatibility note
Review changes
deprecate layout initializer, set collection view on assignment

Also updated documentation to reflect these changes and add a warning
about not reusing layouts for different collection views.
update demos for the changed API
Merge pull request #97 from jwilling/deprecate-layout-initializer

Deprecate layout initializer
Merge pull request #95 from kettch/master

Adding dequeue from Nib
don't use the deprecated initializer in included layouts
Merge pull request #99 from jwilling/override-init-layout

Fix layout initializers
Merge branch 'master' of https://github.com/jwilling/JNWCollectionView