Add computable property. Closes #1

Allows this:

```javascript
var boundObject = Bindable.Object.create({
  firstName:"John",
  lastName:"Doe",
  fullName:function(){
    return this.get('firstName') + " " + this.get('lastName');
  }.property('firstName', 'lastName');
});

console.log(boundObject.get('fullName')); // John Doe

var html = "<p>" + boundObject.bind("fullName") +"</p>";
document.body.innerHTML = html;

boundObject.set('firstName', 'George'); // DOM Updates

```