asString - useful for binding to UI elements that deal only in strings

The most compelling use case for this macro is as follows

````javascript
export default Ember.Controller.extend({
   age: 26,
   ageAsString: asString('age')
});
````

**Scenario w/o `asString`**
We bind to the "age" property. Since HTML inputs deal only
in strings, when the user types in this text box, the value of the "age"
property will be updated with a string value (changing its type)

In this case, "age" changes from a numeric value to a string value
````handlebars
{{input type="text" value="age"}}
````
**Scenario w/ `asString`**
We bind to the "ageAsString" property. The HTML input updates
the "ageAsString" property when the user types in the text box, and
the value of the "age" property is updated (and it's type is respected).

In this case, "age" remains a numeric value.
````handlebars
{{input type="text" value="ageAsString"}}
````

#### Dependent on
#92 :white_check_mark: 

I've changed this to be just `@method asString`.

You can run `ember yuidoc` to generate the documentation. Hopefully this evening I'll improve the the yuidoc integration using broccoli-yuidoc to generate documentation automatically and expose it under the '/docs' url for live editing.