propertyIsModel attribute added for em-select

#73 #72 

new attribute for `{{em-select}}` 

- propertyIsModel : will bind to the model in the collections instead of the value.

```javascript
            {{em-select
            label="Employee"
            property="employee"
            propertyIsModel=true
            content=employees
            optionValuePath="content.id"
            optionLabelPath="content.name"
            disabled=false
            prompt="Please Select"
            }}
```

No the employee object will bind to the actual employee model on the id.