Add increment and decrement to instance

[exos]

Add instance.increment[0] and instance.decrement[1] methods emulating the original behavior

0: http://docs.sequelizejs.com/class/lib/model.js~Model.html#instance-method-increment 1: http://docs.sequelizejs.com/class/lib/model.js~Model.html#instance-method-decrement

[exos]

What's the problem with this PR? I would like to back to use this package for my project dependencies.