fix full_biola_phone_number

Fixed problem when the number you passed in to
`full_biola_phone_number` was nil then it would fall back to using a
method to look up the number. So it wouldn’t return nil back to you,
which probably isn’t what you would expect. Now you need to pass it a
method name instead of a number.
Fix possible confusion with get_phone

It should return `nil` if the the class doesn’t respond to the method
you are trying to send to it instead of just defaulting to
employee_phone or phone.
Merge pull request #24 from biola/fix_biola_phone

fix full_biola_phone_number