Fix generics specification in ExecutionService interface

Currently, some of the ExecutionService interface methods have parameters whose type is specified like

Map<String, Object> variables

However, when attempting to call these with methods with, say, an object of type Map<String,String>, the java compiler will not accept the object without some casting. To fix this, please change the interface to use the wildcard feature of Java 5 generics. This has already been done on other methods. The above would change to:

Map<String, ?> variables

Please also fix the same issue on TaskService (methods setVariables and completeTask). This change would be backwardly-compatible.