Add Sequelize type definition and tests

The bulk of this work was done by samuelneff
<https://github.com/samuelneff> here:
https://github.com/samuelneff/sequelize-auto-ts/blob/master/lib/sequelize.d.ts

I just converted it to an external module, and updated it in areas to
the latest API version (2.0.0 dev13), and improved it a little here and
there and created the tests.