Write first spec test for creating a person "class"
Test pass:  Create person class & initialize with first, last name
Write 2nd spec test for optional Person parameters
Pass test 2: Person can have optional parameters with default values, if needed
Passed test: Can return full name as a function
Pass test: Can convert weight from kilogram to stone
Test pass: User can add a person to their friends
User can remove a person from their list of friends
Fix removeFriend so that user can remove by fullName
Error if user tries to greet empty array of friends
Refactored code final test; throws error if user has no friends