Embedded resource improvements

Some improvements to how embedded resources are handled:

 * Added a `BundleCollection.AddEmbeddedResources` method. Example usage:

```csharp
bundles.AddEmbeddedResources<ScriptBundle>("~/testjs", GetType().Assembly, "MyApp.Resources",
	"js/test1.js",
	"js/test2.js"
);
```

* Modified `ResourceAsset` to inherit from `AssetBase` so transforms are applied.