Add support for external resources in the src array

It's my understanding that the appcache can also cache external resources, however when you add an external resource to the src array, e.g.

```
manifest : {
    generate : {
        src : [
            "http://fonts.googleapis.com/css?family=Black+Ops+One"
        ]
    }
}
```

it doesn't add the url to the explicit cache.  This pull requests fixes that by looking at the *file.orig.src* and seeing which files are external.  It then adds the urls to the explicit cache after the local files have been processed.