Support for composite attributes

Hi,

please find attached below a functionality to pull CompositeData via REST.

This now works:

```bash
curl  -s -H 'Accept: application/json' ".../domains/java.lang/mbeans/type=Memory/attributes/HeapMemoryUsage"

{"label":"HeapMemoryUsage"}
```

```bash
curl  -s -H 'Accept: application/json' ".../domains/java.lang/mbeans/type=Memory/attributes/HeapMemoryUsage/committed"

{"label":"committed","value":"1134034944"}
```