New feature: Store support for Select editor

This will allow the `Select` editor to request data from a server. 

Instead of providing values for the attribute using the `Select` editor, a store
attributes should be provided (with url). 
```javascript
{
    "code":"example",
    "label":"example",
    "attributes":[
        {
            "type":"string",
            "label":"Option",
            "code":"option"
        },
        {
            "type":"text",
            "label":"Select",
            "code":"select",
            "required":true,
            "array":false,
            "emptyValueLabel":"-- Please select --",
            "store": {
                "url" : "../../dijit/tests/_data/{option}",
                "sort" : false
            }
        }
    ],
    "type_property":"ext_type"
}
```
The url can have variables inside it that correspond to other attributes of the same object, from the example above, the value of the option attribute will replace `{option}`
```javascript
{
	option: "states.json",
	select : "LA"
}
```
Which makes the url =  `../../dijit/tests/_data/states.json`

Check out the test case, at `gform/tests/test_primitive_select_store.html`

TO DO: Add Store support to `CheckedMultiSelectAttributeFactory` and `CheckedSelectAttributeFactory`

![gform_1](https://cloud.githubusercontent.com/assets/2190455/2682428/cf87577e-c19a-11e3-9d1e-e47046670b99.png)