Renaming SourceFilteringPersonRegistry to InMemoryPersonRegistry.

Ultimately the filtering is happening in-memory, and this feels more important in describing the class than the fact that it uses a collaborator to get all the people.
Moving the birthday check to the Person object.

This removes the need for an accessor method on the Person, which feels right to me.
Updating note
Ignoring birthdays.txt
Extracting a Name value object class
Replacing String fields in Person with Name value object