[Google] Refactor events

[Taluu]

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.

• Split Google's Event object
• Make an abstract
• Refactor a little bit the statics (while I'm at it)
• Implement a normal event
• [ ] Implement a recurring event Will be done in another pr, as it is a huge thing by itself
• 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 :(

[Taluu]

I'll do the whole thing about the recurring events a bit later, as it needs a bit of work (parsing, ... etc)