birthday default data

Created by: agraubert

Birthdays are stored in the form {userID: {'day':day, 'month':month, 'year':year}}

It would be better if Beymax prefilled his own data when the database is already blank (only if another user adds their birthday). In that case, the userID for the bot instance can be obtained from self.user.id and his birthday is hard-coded as May 6th, 2017 (the day the repo was created)

Additionally, this should wait for #7 (closed) because self.users[] will not capture the bot itself