Autoresize contact picker when data is reloaded or contacts are added or removed programmatically

Fixes #35 

* Change the willChangeContentSizeFrom:to: to not include the from value since it is invalid on data reloads or programmatic removal or addition of selected contacts
* Remove last content size logic from flow layout. If we want to be smart about when we invoke ‘willChangeContentSizeTo:’, we’ll need to find a better way to track content size changes
* This change removes the filtration of content size changes so that all changes are propagated, including ones that don’t cause a height change
* Update the sample application to include a button for clearing contacts and adding a group of selected contacts