Major refactoring of default cloudlet callbacks  (!!! The code is currently broken !!!)

* Add the `DefaultCloudlet` class to be used as base class for all cloudlet "holder" classes.

* Add the `DefaultCloudletContext` class with support for:
  * easy creation of the "usual" connectors by removing the boiler-plate code (see the `create*Connector` variants);
  * add `logger`, `transcript` and `exceptions` that should be used in all cases;

* Update the existing `Default*Callbacks`:
  * add mandatory constructor taking as argument the `CloudletController`;
  * deprecate the usage of `logger`, `transcript` and `exceptions` (use the ones in the context);
  * refactor the `handleUnhandledCallback` and `traceCallback` methods;

* Refactor existing cloudlet examples according to the above enhancements.

* Remove some obsolete cloudlet examples (`SimpleLogging`, `UserLogin`, etc.)
* Remove all JavaDoc comments (they are out-dated and should be rewriten).