Add observable class field support

[cherifGsoul]

For example:

class Person extends ObservableObject{
		greetings = 'Hello';
		static get props() {
			return {
				greetings: 'Bonjour'
			};
		}
}

const cherif = new Person();

cherif.on('greetings', function (ev, newVal, oldVal) {
	    // it should be observable, handle change here
});

// Property change trigger the handler
cherif.greetings = 'Hola';

For canjs/can-observable-mixin#79

[phillipskevin]

I think this should be good:

ObservableObject class fields are also observable.

[cherifGsoul]

@phillipskevin The docs are updated, can you review one more time?

[phillipskevin]

Looks good @cherifGsoul!