Support frozen objects

[andykog]

This code:

var arr = mobservable.observable([]);
var person = Object.freeze({ name: 'bob' })
arr.push(person)

throws error Cannot define property:$mobservable, object is not extensible.

This also prevents from pushing react elements into mobservable array:

buttons.push(<button title="test" />);

[mweststrate]

Thanks for the addition!

[mweststrate]

Released as part of version 1.2.4