Remove old prepre method in example user migration. It was setting ->created=0 instead of current time.

Add dedupe() method for a field mapping. This assures that the value is unique, even if source data contains duplicates. Commonly used for usernames. Uniqueness is currently achieved by appending a number (e.g. example1). In the future we can expand to custom replacement patterns such as email address: sally1@example.com. Dedupe can be used on any mapping.

Allow individual migrations to have prepare methods. These are called last by MigrateDestinationEntity::prepare. Illustrate this feature in the BeerUser migration by converting a Denver ISO time to unix timestamp.

#737174 by moshe weitzman. Set needs_update=1 in mapping table when a referenced entity has not yet been created. useful for articles with 'related articles' Field, for example.

Modernize filefield handler so it has an arguments method ... Also comment out pid in BeerComment migration since it is throwing an error which I can't debug right now.