Event model init.
Added an owner to event - manually by specifying a foreign key, because
belongs_to doesn't directly support aliases.
Added invitation.
Added some Date related stuff. Had to call it "SpoodleDate", because Date
is a reserved word. I didn't add any migrations to add columns but instead
edited old ones, sorry.
Added "availability" as a way of expressing "User X is available on date Y
to participate in event Z".
Revert "Added "availability" as a way of expressing "User X is available on date Y"

This reverts commit 25192f9e133152afab3f8d32f0f1b05fa2b2f170.
Fixed n:m map between user and spoodle_date with join table withou
explicit model. The n:m map between user and event still has an explicit
model, because I think an invitation can contain text (ie. own
attributes). Even "Availabilities" (user-date map) could contain
attributes (ie. priority etc.) but let's think about that later.
Forgot to add something.
Revert "Forgot to add something." ("Verschlimmbesserung")
This reverts commit 13f08cc7c5fb43862b02b652f33948b746ffa359.
Fixed some stuff and refactored some stuff (ie. deleted a join table,
which I generated with the rails 3 generator and used the rails 4 way
instead.)
Added some seeds. Enabled indexes for spoodle_dates_users.
Merge branch 'feature/eventmodel' into dev