Big refactoring of proxy code.

Big refactoring of proxy code.
 * Decorator-based serverside dispatch code
   which is easier to maintain
 * Automatic proxy generation using introspection on
   abstract interface (Proxy/ExperimentController) and
   serverside dispatch code.
 * Tried to maintain meaningful tracebacks
 * No more repeated code - stuff is only written once,
   so changes are easier to apply.
 * Changed marshalling for setters to use pickling instead
   of adding a fictious (and limited) type parameter.
 * Fixed some interface mismatches in the process