Support for HtmlContent for operations

- added support of HtmlContent for JMX operations (was supported just for attributes). 
- introduction of a new interface InputStreamContent. If a attribute or operation returns this interface, the browser will download all data from this input stream. I added this so now I can create **AND** automaticall download a java heap dump from within JMiniX. Sadly I found no way to force a filename for the download (a filename can be specified using the Disposition class, but sadly gets ignored).
- I also added a little sample MBean that demonstrates the usage of HtmlContent and InputStreamContent. This sample MBean can be accessed when running the sample org.jminix.console.Main application.