Use new propertyAlias instead of custom property alias for validation

This should fix the concerns mentioned in #163 by using the ```propertyAlias``` for property identification during validation instead of a custom property alias based on the deprecated ```umbracoPropertyAlias```.

Not sure why the PR lists the files from my forward merges of master as changed (@leekelleher's latest changes) - I've only changed ```app/directives/archetypeproperty.js``` to fix this :)