[Google] Recurring events

Handling Recurring event for Google Adapter. This needs a little refacto of the google events system, in order to handle properly the recurring events... and their cancellation.

When the flag `showDeleted` is set to `true`, the deleted events (normal & recurring) are shown as the normal events.

If it is set to `false`, only cancelled instances of a recurring events, normal events, and recurrent events are shown... but the cancelled recurring events are somewhat incomplete (missing a bunch of properties which may seem important, such as the creator of the event, the start & end orginal dates, ... etc).

Fucking Google, yeah.

- [x] Split Google's Event object
- [x] Make an abstract
- [x] Refactor a little bit the statics (while I'm at it)
- [ ] Implement a normal event
- [ ] Implement a recurring event
- [ ] Implement a cancelled recurring event when it is truncated (phew)

The tests should be added a little bit later when I'll have some time ahead :(