Added support for Array Change subscriptions. Added tests to the test_observableArrays() in knockout-tests.ts.

I added support for Array Change Subscriptions (issue link: https://github.com/borisyankov/DefinitelyTyped/issues/2933)
Hi @DenEwout,

First up thanks for contributing!  Could you tell me why you've dropped the generic here please?  I would have thought that switching to Object would lose useful information...