Add support for accepting invitations

[blueleader07]

Accepting invitations is not supported (currently) by the app.

The iOS version does support accepting invitations -- but there is a slight difference in the iOS and Android versions. In Android-land you are able to use your existing calendar app(s) to manage events. Apple is more restrictive -- I could not rely on existing calendar app(s). So, I built a calendar view inside Team Calendar Sync for the iOS version. Because I have control over the calendar I could support accepting invitations. Android is a bit trickier -- since I don't have control over the calendar apps. When I last looked at supporting accepting invitations in Android some calendar apps supported it while others did not. Therefore, I did not implement it. I will take another look and see if it can be done.