Avoid errors when using aliases in calendar invites

[lightheaded]

If the calendar ID (i.e. the primary email) doesn't exactly match the invitee's email, gcalcli agenda --nodeclined will throw an error.

This change changes makes use of the 'self' property of the attendee to determine identity instead of comparing strings, thus enabling support for alias-based invitations.

Now it's possible for firstname.lastname@domain.com to be invited as firstname@domain.com and still use gcalcli agenda --nodeclined.

https://github.com/insanum/gcalcli/issues/620

[lightheaded]

Hi @insanum! Would you mind merging it?

[dbarnett]

Fixed in #701 instead.