Add the ability to add a new gift idea to a birthday

FIXME: The page needs to be refreshed manually for the list to be
updated
Merge pull request #25 from sebn/add-gift-idea

Add the ability to add a new gift idea to a birthday
WIP: Set up birthday person change UI (not core)
Create feature function skels
Change person, but still show the old description
Fix core/birthdays tests to enfore birthday person
Ensure all birthday properties are stored
Update birthday description on person change
Birthday person creation & change
Merge pull request #24 from sebn/change-birthday-person

Change birthday person
Homepage redirects according to first use detection (to be implemented)
Use lodash to implement core/utils
Add and use utils/objects.merge function
Introduce core/utils/functions.identity()