Integrate with Google Calendar API

[shubhpatel108]

One use case is covered in #78

[paulinahaduong]

Potentially could address https://github.com/berkmancenter/bookanook/issues/28 with invites through gcal instead.

[shubhpatel108]

Integration architecture:

Scenarios:

1. Every Google user have a calendar list where he can add personal calendars as well as other public/permitted calendars. In our case, every Nook will have its own calendar, primarily attached to the Admin who created the nook, i.e. this calendar will be added to his/her GC list. Other admins may or may not add this calendar to their GC list. If possible, we can have the calendar detachable from the owner too.
2. Many admins can add a nook’s calendar to their calendar list as shown in the diagram.
3. As an admin could be associated with many locations, he/she can add calendars of nooks related to other locations too.
4. For a confirmed Event1, the requestor User1 will able to add it to its personal Google calendar. This will be done on demand, for every reservation separately. We can have a common sync functionality too.
5. A confirmed Event2 is created by User1 but is not added to his own personal calendar. However when he invites User2 (using Google Contacts APIs), the latter accept it and Event2 gets added to his/her calendar. This is an incremental goal and can be implemented only if time permits.
6. Delete the GC event if the status is changed back from confirmed to any other. This is because the GC events are always assumed to be confirmed.
7. A user can opt for pre-event alerts (emails, pop-ups and SMS) from Google.
8. Update the GC event information like title and description when an event is updated on our app.

quoted from my GSoC '16 proposal.