Add id attribute to resources to allow overriding in ApplicationResources.groovy

Per [my note](https://github.com/sheehan/grails-handlebars-resources/issues/5#issuecomment-12331805) on [Issue 5](https://github.com/sheehan/grails-handlebars-resources/issues/5) -- I added IDs to the JS resources in `HandlebarsResources.groovy`, so they can be referenced with an `overrides` block in `ApplicationResources.groovy`:

```groovy
overrides {
    handlebars {
        resource id:'handlebars', disposition: 'head'
    }
}
```