Refactored Event to store a person instance attribute

[romanZupancic]

Actually makes these series of classes usable, but most of the event implementation still sucks. I will work on adding #16 in a future pull request, along with any tagging features we want implemented for events.

For this pull request: Do we want to get rid of the event heirarchy altogether, and just use tags instead on a single concrete event class?

[nathan-hansen]

We can't entirely remove the hierarchy, since anniversaries function differently than birthdays (reminding at 1, 3, and 6 months and then each following year compared to just every year). for now i think we put anniversaries on the backburner though.

[romanZupancic]

We can't entirely remove the hierarchy, since anniversaries function differently than birthdays (reminding at 1, 3, and 6 months and then each following year compared to just every year). for now I think we put anniversaries on the backburner though.

That is a very good point. I'm tagging #16 for when we implement recurring reminders.