Extensions to improve API coverage

Hello!
  Lately I've been working a bit on some extensions to susestudio-lib-java in order to extend API coverage. These are just the first changes; I plan to add some more in the near future.

It would be great if you had a little time to review my patches! Please send me any comments you have.

I'm particularly interested in knowing if you agree on adding a "StudioException" that is thrown when a REST method results in an error - that can be almost anywhere, so my proposed patch breaks the current API. Nevertheless it seems to me that an exception is the cleanest and most effective way to handle such cases, so I assumed that an API change is acceptable.

Another assumption I made is that classes in com.studio.studio.suse.client.net (RequestFactory and StudioConnection) are really internal classes despite being declared as public. I don't think users should invoke their methods directly anyway, so I felt free to modify their interfaces as well.

Changes can obviously be reverted if those hypotheses turn out to be wrong.

Greetings,

Silvio