Rename birthday -> event
Rename birthday -> event

General renaming/rewording of var/function-names and comments
in preparation for the upcoming refactoring, to hopefully make
that diff easier to read. Also includes a small comment
grammar-fix at line 344.

baatochan/GoogleContactsEventsNotifier,GioBonvi/GoogleContactsEventsNotifier,rowanthorpe/GoogleContactsEventsNotifier