[*] transaction handling of remove/update

I realized that there are some strange things for me in exception handling of transaction. Please look at the code I've implemented as I think that all update operation has to be in transaction by default or at least there has to be clear option how to handle rollback operation. I considered usage of RDFTransaction model, but I found that I don't know how/when to use rollback. Update operations don't throw any rollback specific (checked) exception to handle, when rollback should be called. I realized that remove() method had different transaction handling (no begin and commit method) than update(). This pull request bit mixes both approaches(calling direct RepositoryConnection.begin() vs RDFTransaction.begin()). Therefore I suggest to throw some checked exception when update operation failed to use RDFTransaction.rollback(). Or your intention was to catch unchecked RepositoryException? This pull request should be updated according to your decision of transaction handling.