Add toString and toText method

I've added in the toString and toText methods. Blaze calls these internally when an object is used in a template as a value or in an object of attributes. 

examples:

```js
var name = new ReactiveProperty('Joe');
var attrs = {
  id: new ReactiveProperty('randomId')
};
Template.hello.helpers({
  name: function() { 
    return name; 
  },
  attrs: function() {
    return attrs;
  }
});
```

```handlebars
<h1 {{attrs}}>
  {{name}}
</h1>
```