Create Abstract for Calendar and Event model class

[eldo-greshard]

I create 2 new abstract model class for calendar and event so I can extend the calendar and event model with another model like django-multitenant. and I create another sample project integrate with fullcalendar vue.

[coveralls]

Coverage increased (+0.2%) to 80.111% when pulling 0b54897 on karmael:develop into f240c7c on llazzaro:develop.

[eldo-greshard]

continuous-integration/travis-ci/pr — The Travis CI build failed
i tried to test with tox in my local, it's pass all the test.