Use of static instead of self for better extensibility

[bilogic]

1. We are extending the GoogleCalendarFactory class in order to write to multiple calendars instead of just 1 fixed calendar
2. However, most of the calls in the class uses self:: instead of static::, thus, we had to overwrite several methods that look identical in text form
3. Possible to change from using self:: to static::? I'm more than happy to file a PR

[freekmurze]

Feel free to PR this