Permissions improvements

In my own project I have to check permissions on calendar, not on each event instance. 
From current upstream branch version I can't for example check when user has permission to add event in particular calendar. This PR add new setting - CHECK_CALENDAR_PERM_FUNC and deprecate CHECK_PERMISSION_FUNC for using new CHECK_EVENT_PERM_FUNC. This seems more reasonable name for me.

Also this PR fixes some test failures and fixes to setup.py introduced by https://github.com/llazzaro/django-scheduler/pull/38